feat(#836): no-LLM duplicate-issue detection + challenge + 1-day auto-close (#843)

* feat(#836): no-LLM duplicate-issue detection + challenge + 1-day auto-close

Adds a deterministic (no-LLM) duplicate-issue governance lifecycle:

- scripts/issue-dedupe.cjs: pure, unit-tested module (tokenize, Sørensen–Dice
  title similarity, scoreCandidates, renderChallengeComment, shouldClose) with
  fail-safe destructive-action guards.
- duplicate-check.yml (issues:opened): scores new-issue title against open
  issues, posts a challenge comment + applies the pending `possible-duplicate`
  label on a clear match.
- duplicate-sweep.yml (daily cron): closes possible-duplicate issues whose
  challenge comment is >24h old with no human reply and no 👎 veto; honors
  exempt labels; re-checks the label immediately before close (TOCTOU guard);
  strips the label on close to avoid reopen loops.
- remove-duplicate-label.yml (issue_comment:created): clears the label and
  applies needs-maintainer-review when any human responds.
- bug_report.yml / docs_issue.yml: add the required "I searched existing
  issues" preflight checkbox so all five forms force a pre-search attestation.
- docs/agents/triage-labels.md: document the label + lifecycle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#836): add changeset fragment for duplicate-issue detection

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/eager-birds-squeak.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 843
+---
+Issues are now checked for duplicates when opened: a no-LLM title-similarity check posts a challenge comment and applies a `possible-duplicate` label when a new issue closely matches existing open ones. Flagged issues that go unanswered for 24h are auto-closed as duplicates (reply, or react 👎 to the bot comment, to keep one open); a reply clears the label and routes to `needs-maintainer-review`. (#836)