Auto-build bundled plugins on install (#8254)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Plugins extend the server with worker/UI surfaces, and bundled local
plugins under `packages/plugins/**` ship as TS source — their compiled
`dist/` is not checked in
> - On a fresh checkout, installing a bundled local plugin via the
in-app **Install** button failed because `paperclipPlugin.manifest`
points at `./dist/manifest.js`, which does not exist until the package
is built
> - The error surfaces as `Package … does not appear to be a Paperclip
plugin (no manifest found)`, which is misleading — the manifest is real,
the dist is just missing — and forces every contributor to run `pnpm
--filter … build` by hand before the bundled-plugin installer works at
all
> - This pull request teaches the install path to detect that case and
run the package's build (plus standalone runtime bootstrap for plugins
outside the root workspace) before manifest resolution, gated by a kill
switch and a bounded timeout
> - The benefit is bundled plugins like
`@paperclipai/plugin-workspace-diff` install in one click on a fresh
checkout, with a clear error message and manual fallback when the
autobuild itself fails

## Linked Issues or Issue Description

No existing GitHub issue. Underlying bug, following the bug-report
template:

**What happened?**
Installing a bundled local plugin from a fresh checkout fails with
`Package @paperclipai/plugin-workspace-diff at
packages/plugins/plugin-workspace-diff does not appear to be a Paperclip
plugin (no manifest found)`. The manifest is declared in `package.json`
(`paperclipPlugin.manifest = ./dist/manifest.js`) but `dist/` is not
built/committed, so the loader cannot find it.

**Expected behavior**
Clicking **Install** on a bundled plugin builds it if needed and
registers it, without a manual build step.

**Steps to reproduce**
1. Fresh checkout of `master`
2. Start the server, open Plugin Manager
3. Click **Install** next to `@paperclipai/plugin-workspace-diff`
4. Observe the "no manifest found" failure

**Scope**
Same failure mode affects every bundled plugin without a checked-in
`dist/` (`plugin-llm-wiki`, examples, sandbox-provider plugins, etc.).

## What Changed

- `server/src/services/plugin-loader.ts`: added
`ensureLocalPluginBuilt(packageRoot, pkgJson)` — when the package lives
under `packages/plugins/**` and its declared paperclipPlugin entrypoints
(`manifest`, `worker`, `ui`) are missing, run `pnpm --filter <name>
build` (and a standalone runtime-deps bootstrap for plugins outside the
root pnpm workspace) before manifest resolution
- `server/src/routes/plugins.ts`: invoke the autobuild from the
local-path install path; surface a `hasBuiltEntrypoints` boolean on the
`AvailableBundledPlugin` listing; invalidate the bundled-plugins cache
after a successful install so a freshly built plugin no longer reports
`hasBuiltEntrypoints: false`
- `ui/src/api/plugins.ts` + `ui/src/pages/PluginManager.tsx`: type and
consume `hasBuiltEntrypoints` so the installer can show that an
autobuild will run on install
- `server/src/__tests__/plugin-install-autobuild.test.ts`: new suite — 9
tests covering success, kill-switch, build failure, timeout, manifest
still missing after build, standalone variant, and the existing
`plugin-routes-authz` listing assertion
- `doc/plugins/LOCAL_PLUGIN_DEVELOPMENT.md`: documents the autobuild,
the `PAPERCLIP_DISABLE_PLUGIN_AUTOBUILD=1` kill switch, and the manual
fallback command
- Detect the autobuild timeout via the child-process `killed` flag
rather than string-matching the error message, so the "after timing out"
context is actually emitted

Knobs:

- `PAPERCLIP_DISABLE_PLUGIN_AUTOBUILD=1` — skip autobuild entirely;
restore prior behavior
- Build timeout: 120s, with a clear error that points at the manual
`pnpm --filter <name> build` recovery command

## Verification

- `cd server && pnpm vitest run
src/__tests__/plugin-install-autobuild.test.ts
src/__tests__/plugin-routes-authz.test.ts` → 44/44 pass
- End-to-end on a clean checkout: `rm -rf
packages/plugins/plugin-workspace-diff/dist`, invoke
`ensureLocalPluginBuilt()` against the real package, all declared
entrypoints (`dist/manifest.js`, `dist/worker.js`, `dist/ui/index.js`)
regenerated. The original `no manifest found` symptom no longer
reproduces.

## Risks

Low. The autobuild only fires when (a) the package sits under
`packages/plugins/**`, (b) at least one declared entrypoint is missing,
and (c) the kill switch is not set. In a packaged production server the
`packages/plugins/**` path does not exist on disk, so the helper
short-circuits and never shells out to `pnpm`. Failures from the spawned
build are surfaced as an install error with the exact manual command to
retry, so the worst-case is the same UX as before plus a clearer
message.

## Model Used

Claude Opus 4.7 (claude-opus-4-7), extended thinking enabled, tool use
(filesystem + bash).

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Paperclip <noreply@paperclip.ing>

doc/plugins/LOCAL_PLUGIN_DEVELOPMENT.md

@@ -116,7 +116,7 @@ What that means in practice:
 - **Without `pnpm dev`:** the watcher only fires on `dist/*` changes. If you stop the watch build, source edits do not reach Paperclip. Restart `pnpm dev` (or run `pnpm build` once) before expecting changes.
 - **`node_modules`, `.git`, `.paperclip-sdk`, and other dotfolders are ignored.** Adding a dependency requires the new code to actually be imported and rebuilt before the worker sees it.
 
-The server never compiles plugin source for you. The package's own build scripts own that step.
+The package's own build scripts still own compilation. Paperclip does not compile arbitrary local-path plugins for you. The exceptions are bundled plugins inside the Paperclip repo under `packages/plugins/`: workspace packages auto-build once with `pnpm --filter <package> build`, and standalone sandbox-provider packages under `packages/plugins/sandbox-providers/` first bootstrap package-local dependencies with `pnpm install --ignore-workspace ...` and then run `pnpm build` in place. Set `PAPERCLIP_DISABLE_PLUGIN_AUTOBUILD=1` in the server environment to disable those fallbacks.
 
 ## Local path plugins vs npm packages
 
@@ -136,7 +136,7 @@ When you are done iterating locally, publish the package and reinstall the npm-p
 
 ## Troubleshooting
 
-- **`Plugin install returned no plugin record` or `error` status.** Run `paperclipai plugin inspect <key>` for the last error. The most common causes are (1) the plugin has not built yet — run `pnpm dev` or `pnpm build` first, (2) the `paperclipPlugin` entries in `package.json` point at files that do not exist on disk, or (3) the manifest failed validation. The Paperclip server log has the full validation error.
+- **`Plugin install returned no plugin record` or `error` status.** Run `paperclipai plugin inspect <key>` for the last error. The most common causes are (1) the plugin has not built yet — run `pnpm dev` or `pnpm build` first, (2) the `paperclipPlugin` entries in `package.json` point at files that do not exist on disk, or (3) the manifest failed validation. Bundled repo plugins may auto-build once during install, but external local-path plugins still require you to build them yourself. The Paperclip server log has the full validation error.
 - **Edits do not seem to reload.** Confirm `pnpm dev` is still running and writing to `dist/`. If you renamed entry files, update the `paperclipPlugin.manifest` / `paperclipPlugin.worker` / `paperclipPlugin.ui` fields in `package.json` so the watcher targets them.
 - **Worker restarts but UI is stale.** Hard-reload the page. If you want HMR, run `pnpm dev:ui` and set `devUiUrl` in your manifest to `http://127.0.0.1:4177` during development.
 - **Path arguments fail on Windows.** Quote paths that contain spaces, and prefer absolute paths over `~`-prefixed paths in non-bash shells.