feat(FN-4774): complete Step 1 — add duplicate-detection post-mortem

Fusion-Task-Id: FN-4774
Fusion-Task-Lineage: f3b81550-fb23-441e-aee7-44ffec63d03d

docs/triage-duplicate-detection-postmortem.md

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+# Triage Duplicate Detection Post-Mortem (FN-4774)
+
+## Summary
+Between 14:29 and 16:05 on 2026-05-16, three independent tasks were filed and executed for the same defect: rebase-merge done-task diff truncation in `packages/dashboard/src/routes/register-session-diff-routes.ts` (with related tests in `packages/dashboard/src/__tests__/routes-diff.test.ts`). This duplicated implementation and review effort.
+
+## Timeline
+- **FN-4726**
+  - Created: **14:29**
+  - Merged: **14:52**
+  - GitHub tracking: **#400**
+  - Scope touched: `register-session-diff-routes.ts`, `routes-diff.test.ts`
+- **FN-4734**
+  - Created: **14:54**
+  - Finalization: **15:33** (auto no-op finalize)
+  - GitHub tracking: **#414**
+  - Scope touched: same target files
+- **FN-4741**
+  - Created: **15:04**
+  - Merged: **16:05**
+  - GitHub tracking: **#424**
+  - Scope touched: same target files
+
+## Root cause
+1. **Triage duplicate scan blind spot:** `fn_task_list` intentionally excludes `done` tasks (`tasks.filter((t) => t.column !== "done")` in `packages/engine/src/triage.ts`), so recently completed fixes were invisible to the duplicate check.
+2. **No triage keyword search tool:** Triage had no exposed keyword search over full task history (including done/archived), despite `TaskStore.searchTasks(...)` already existing and backed by FTS5.
+3. **GitHub issue duplication gap:** Tracking issue creation did not include a check for existing closed issues covering the same files/keywords.
+
+## Decision
+1. **Add `fn_task_search` triage tool** that wraps `store.searchTasks(...)` with defaults that include historical tasks (`includeArchived: true`, done tasks included by default).
+2. **Strengthen duplicate-check prompt guidance** so triage must run both list and keyword search (multiple phrases from title/description/file paths/symptoms/symbol names), then inspect likely matches before filing.
+3. **Record GitHub-side dedup as follow-up work** (new task) rather than expanding this fix’s scope.
+
+## Acceptance-criteria mapping
+1. **Document what happened and why:** covered by timeline + root-cause analysis above.
+2. **Close triage duplicate-detection gap:** addressed by adding `fn_task_search` and prompt instructions to query done/archived history.
+3. **Preserve scope discipline:** GitHub-side dedup is explicitly deferred into a follow-up task instead of being implemented here.