feat(commitperclip): widen linked-issue gate, add dedup-search check (#7632)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Contributor onboarding leans on a small set of CI gates
("commitperclip") that read PR metadata and enforce the rules in
CONTRIBUTING.md
> - We recently landed two new contributor rules: every PR must either
link an existing issue or inline an issue-template-shaped description,
and the author must affirm they searched for duplicate PRs first
> - The existing `check-pr-linked-issue` gate only accepted
`Fixes/Closes/Resolves #N`, and there was no gate at all for the
dedup-search affirmation, so the new rules were unenforced
> - This pull request widens the linked-issue gate (accept `Refs #N`,
accept inline template-shaped descriptions) and adds a new dedup-search
gate wired into `run-quality-gates`
> - The benefit is that the rules we ask contributors to follow are now
mechanically enforced, lowering review noise without raising contributor
friction

## Linked Issues or Issue Description

Refs #4260 — the PR cited in the original rule discussion, whose body is
the canonical example of the inline-issue-description shape this gate
now accepts.

**Problem or motivation**

The repo recently adopted two contributor rules: (1) link the issue your
PR fixes or inline a description in the issue-template shape, and (2)
confirm you searched for duplicate PRs before opening one. Neither rule
was enforced — the existing linked-issue check only matched
`Fixes/Closes/Resolves #N`, and nothing looked for the dedup-search
affirmation. Reviewers had to remember and re-state the rules by hand on
every PR.

**Proposed solution**

Widen `check-pr-linked-issue.mjs` and add a new
`check-pr-dedup-search.mjs` so commitperclip enforces the rules already
documented in CONTRIBUTING.md and the PR template:

- Accept `Refs #N` as a valid link verb alongside `Fixes`, `Closes`,
`Resolves`.
- Accept a PR body with an inline issue-template-shaped description (≥3
fields matched against the bug, feature, or adapter templates) as a
valid alternative to a linked issue.
- New `check-pr-dedup-search.mjs` looks for a checked checkbox affirming
the author searched for similar/duplicate/prior PRs, wired into
`run-quality-gates.mjs` with the same skip/override semantics as the
other gates.

**Alternatives considered**

- A single regex over the whole PR body looking for issue-template field
names — rejected, too brittle and gave no useful error message when it
failed. The per-field counter lets us tell the author exactly how many
template fields we matched and which template they're closest to.
- Making the dedup-search check live inside `check-pr-linked-issue` —
rejected, the two rules are orthogonal and a separate gate gives a
cleaner error message and respects `[skip-quality-gates]` independently.

**Roadmap alignment**

This is contributor-workflow plumbing for rules that already landed on
`master`. It is not a roadmap feature and does not overlap with planned
core work.

## What Changed

- `check-pr-linked-issue.mjs`: accept `Refs #N`; add
`hasInlineIssueDescription` (per-template field counter, ≥3 fields) so a
fully inlined description satisfies the gate.
- `check-pr-dedup-search.mjs`: new gate, looks for a checked
dedup-search checkbox in the PR body, with clear failure guidance.
- `run-quality-gates.mjs`: wire the new gate in alongside the existing
checks.
- `CONTRIBUTING.md`: "Before You Start: Search First" callout pointing
at the PR-template checkbox the gate enforces.
- Tests: unit tests for `Refs #N`, the inline-description threshold
(bolded labels, plain labels, headings), and the dedup-search gate
across checked/unchecked/missing/skip-prefix paths.

## Verification

- `node --test .github/scripts/tests/check-pr-linked-issue.test.mjs` →
all pass
- `node --test .github/scripts/tests/check-pr-dedup-search.test.mjs` →
all pass
- Full repo test matrix: 116/116 pass locally

For the gate itself, this PR exercises both new code paths: the body
inlines a feature-template-shaped description with ≥3 matched fields,
and the dedup-search checkbox below is checked.

## Risks

Low risk. The change is contributor-CI plumbing — no runtime or
migration impact. The widened linked-issue gate is strictly more
permissive (it can only flip prior FAILs to PASS), and the new
dedup-search gate honors the existing `[skip-quality-gates]` prefix, so
an author can always bypass it the same way as the other gates if
needed.

## Model Used

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

## 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>

CONTRIBUTING.md

@@ -14,6 +14,8 @@ Before you start work, **search GitHub** for existing PRs and issues that touch
 
 Duplicate PRs create extra work for reviewers and make merging harder. A 60-second search saves hours later.
 
+Affirm that you did this search by checking the dedup-search box in the PR template (`I have searched GitHub for duplicate or related PRs and linked them above`). Commitperclip checks for this checkbox on non-trivial PRs.
+
 ## Two Paths to Get Your Pull Request Accepted
 
 ### Path 1: Small, Focused Changes (Fastest way to get merged)