Expand plugin host surface (#5205)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The plugin system is the extension boundary for optional product
capabilities
> - Rich plugins need more than a worker entrypoint: they need scoped
database storage, local project folders, managed agents/routines, host
navigation, and reusable UI components
> - The LLM Wiki work exposed those missing host surfaces while keeping
plugin code outside the core control plane
> - This pull request expands the core plugin host, SDK, server APIs,
and UI bridge so plugins can declare and use those surfaces
> - The benefit is that future plugins can integrate with Paperclip
through documented, validated contracts instead of bespoke server or UI
imports

## What Changed

- Added plugin-managed database namespaces and migration tracking,
including Drizzle schema/migration files and SQL validation for
namespace isolation.
- Added server support for plugin local folders, managed agents, managed
routines, scoped plugin APIs, and plugin operation visibility.
- Expanded shared plugin manifest/types/validators and SDK
host/testing/UI exports for richer plugin surfaces.
- Added reusable UI pieces for file trees, managed routines, resizable
sidebars, route sidebars, and plugin bridge initialization.
- Updated plugin docs and example plugins to use the expanded host and
SDK surface.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm run preflight:workspace-links && pnpm exec vitest run
packages/shared/src/validators/plugin.test.ts
server/src/__tests__/plugin-database.test.ts
server/src/__tests__/plugin-local-folders.test.ts
server/src/__tests__/plugin-managed-agents.test.ts
server/src/__tests__/plugin-managed-routines.test.ts
server/src/__tests__/plugin-orchestration-apis.test.ts
ui/src/api/plugins.test.ts ui/src/components/FileTree.test.tsx
ui/src/components/ResizableSidebarPane.test.tsx
ui/src/pages/PluginPage.test.tsx ui/src/plugins/bridge.test.ts` passed:
11 files, 67 tests.
- Confirmed this PR changes 89 files and does not include
`pnpm-lock.yaml` or `.github/workflows/*`.

## Risks

- Medium: this expands plugin host contracts across db/shared/server/ui
and includes a new core migration (`0076_useful_elektra.sql`).
- The plugin database namespace validator is intentionally restrictive;
plugin authors may need follow-up affordances for SQL patterns that
remain blocked.
- Merge this before the LLM Wiki plugin PR so the plugin can resolve the
new SDK and host APIs.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool-enabled shell/git/GitHub
workflow. Context window size was not exposed by the runtime.

## 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 run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] 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
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

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

packages/plugins/sdk/src/ui/hooks.ts

@@ -1,6 +1,8 @@
 import type {
   PluginDataResult,
   PluginActionFn,
+  HostLocation,
+  HostNavigation,
   PluginHostContext,
   PluginStreamResult,
   PluginToastFn,
@@ -115,6 +117,57 @@ export function useHostContext(): PluginHostContext {
   return impl();
 }
 
+// ---------------------------------------------------------------------------
+// useHostNavigation
+// ---------------------------------------------------------------------------
+
+/**
+ * Navigate within the Paperclip host without forcing a full document reload.
+ *
+ * Use `linkProps()` for links so browser-native behavior still works:
+ * modifier-click, middle-click, copy-link, and open-in-new-tab all use the
+ * returned real `href`.
+ *
+ * @example
+ * ```tsx
+ * function WikiSidebarLink() {
+ *   const hostNavigation = useHostNavigation();
+ *   return <a {...hostNavigation.linkProps("/wiki")}>Wiki</a>;
+ * }
+ * ```
+ */
+export function useHostNavigation(): HostNavigation {
+  const impl = getSdkUiRuntimeValue<() => HostNavigation>("useHostNavigation");
+  return impl();
+}
+
+// ---------------------------------------------------------------------------
+// useHostLocation
+// ---------------------------------------------------------------------------
+
+/**
+ * Observe the current host router location.
+ *
+ * Returns a snapshot of the active `pathname`, `search`, and `hash`. The
+ * component re-renders when any of these change (e.g. after the host router
+ * pushes a new entry, or after the browser back/forward gestures). Use this
+ * for URL-driven plugin UI such as a takeover sidebar with section-aware
+ * active state.
+ *
+ * @example
+ * ```tsx
+ * function WikiSection() {
+ *   const { pathname } = useHostLocation();
+ *   const section = pathname.split("/").filter(Boolean).at(-1) ?? "wiki";
+ *   return <div>Active section: {section}</div>;
+ * }
+ * ```
+ */
+export function useHostLocation(): HostLocation {
+  const impl = getSdkUiRuntimeValue<() => HostLocation>("useHostLocation");
+  return impl();
+}
+
 // ---------------------------------------------------------------------------
 // usePluginStream
 // ---------------------------------------------------------------------------