docs(triage): sync README with shipped pipeline; drop plan + research (#480)

The README was drafted as a design spec before implementation. Now
that the pipeline is live and the design has been validated end-to-
end, bring the doc into agreement with the code and retire the two
companion files.

README updates:
- Intro: state the production trigger (`issues: [opened]`) and the
  workflow_dispatch fallback; note v1 is manual-only
- Stage 7 table: reorder by actual priority (drift is no longer a
  top-of-gate veto); drift section rewritten to describe the banner-
  and-candidates-modifier behavior landed in PR #476
- Stage 8a rendered-output example: show the conditional drift
  banner + drift-bridge candidates block that actually render
- Stage 8b reason enum: add `reference-source unavailable` that was
  missing from the list
- Rollout posture: describe the cutover as completed, not deferred
- Implementation layout: drop "during rollout" qualifier; add
  helper-scripts row (validate.sh / drift-bridge.sh /
  suspicious-input-scan.sh / extract-json.py)
- Artifacts list: full set with 14-day retention, not just the
  original four
- Reasons.json SSOT pointer: actual path `.claude/scripts/reasons.json`
  instead of the aspirational `lib/templates/reasons.json`
- Potential future improvements: drop "Cutover to issues:[opened]"
  subsection (done)
- Clean up "v1" usage where it means "first version of the pipeline"
  (confusable with legacy v1 workflow)

Deleted:
- docs/issue-triage/implementation-plan.md — phased build sequence
  is complete; commit history preserves the record
- docs/issue-triage/research-trail.md — design-pass sources are cited
  inline in the README where needed

Workflow banner updated to drop the `implementation-plan.md` pointer.

Co-authored-by: Claude <claude@anthropic.com>

.github/workflows/issue-triage-v2.yml

@@ -20,7 +20,7 @@ run-name: |
 # triage on backfilled issues. v1 (issue-triage.yml) is kept as a
 # workflow_dispatch-only fallback — its `issues` trigger was
 # disabled when v2 took over production routing.
-# See docs/issue-triage/{README.md,implementation-plan.md}.
+# See docs/issue-triage/README.md.
 
 on:
   issues: