fix(agents): prevent backlog groomer from closing PRs as duplicates of their tracking issues

Agent evolver identified a critical systematic pattern:
- Pattern: The backlog groomer was closing PRs as 'duplicates' of their
  linked tracking issues. A PR containing 'Closes #N' was being treated
  as a duplicate of issue #N, when it is actually the implementation
  delivery vehicle for that issue.
- Evidence: At least 12 PRs were incorrectly closed (#1219, #1236, #1247,
  #1269, #1267, #953, #1198, #1220, #1237, #1238, #1246, #1248) — all
  with the same 'Duplicate Detected' comment pattern from groomer-1.
- Fix: Added explicit instructions to skip PRs during duplicate detection,
  added a guard in the analysis loop pseudocode, and added a rule in the
  Important Rules section.

This change requires human approval before taking effect.

ISSUES CLOSED: #2180

.opencode/agents/ca-backlog-groomer.md

@@ -87,8 +87,13 @@ LOOP FOREVER:
     all_prs = query Forgejo for all open and recently closed PRs
 
     # ── Step 2: Run all analysis passes ──────────────────────────
+    # IMPORTANT: Filter out pull requests before duplicate detection.
+    # PRs are NOT issues — they deliver code for issues. A PR that
+    # "Closes #N" is the implementation of #N, NOT a duplicate.
+    issues_only = [i for i in all_issues if i.pull_request is None]
+    
     findings = []
-    findings += check_duplicates(all_issues)
+    findings += check_duplicates(issues_only)  # Issues only, never PRs
     findings += check_orphans(all_issues)
     findings += check_stale_issues(all_issues)
     findings += check_label_quality(all_issues)
@@ -153,14 +158,25 @@ LOOP FOREVER:
 
 ### 1. Duplicate Detection
 
-Compare every pair of open issues for similarity:
+Compare every pair of open **issues** for similarity:
 - **Title similarity** — issues with very similar titles (>80% word overlap)
 - **Description similarity** — issues describing the same work
 - **Same branch name** — two issues with the same branch in metadata
 
+**CRITICAL: Pull Requests are NOT duplicates of their linked issues.**
+A PR that contains `Closes #N` in its body is the **implementation delivery
+vehicle** for issue #N — it is NOT a duplicate. Never close a PR because it
+references or implements a tracking issue. Duplicate detection applies ONLY
+to issue-vs-issue comparisons, never to PR-vs-issue comparisons.
+
+When scanning for duplicates, **skip all pull requests entirely**. Only
+compare issues (items where `pull_request` is null in the Forgejo API
+response) against other issues. PRs have a different lifecycle and purpose
+than issues — they deliver code, while issues track work.
+
 **Action:** Post a comment on the newer issue noting the potential duplicate.
 If confidence is very high (same branch name, near-identical title), close
-the newer issue as duplicate.
+the newer issue as duplicate. **Never close a pull request as a duplicate.**
 
 ### 2. Orphan Detection
 
@@ -403,6 +419,10 @@ No exceptions — every comment, every issue body, every PR description.
 - **Be conservative with closes.** Only close issues when you are CERTAIN
   the work is done or it is clearly a duplicate. When in doubt, comment
   instead of closing.
+- **NEVER close pull requests as duplicates of their tracking issues.**
+  A PR that says `Closes #N` is the implementation of issue #N, not a
+  duplicate. PRs and issues serve fundamentally different purposes. Only
+  apply duplicate detection to issue-vs-issue comparisons.
 - **Be helpful, not noisy.** Don't post comments on every issue every cycle.
   Only post when you find an actionable problem.
 - **Respect human decisions.** If an issue has a comment from a human