Authoring-assist: promote to first-class tool with non-blocking PR advisory

Promotes the authoring-assist prototype to a first-class BCQuality feature that
reviews knowledge-article front-matter and proposes missing routing `signals:`
(raise triggers and `effect: suppress` suppressors). The suggestion is ADVISORY
ONLY: it never fails a build; authors apply it via a normal PR under existing
R29 + CI + CODEOWNERS review.

Feature:
- tools/Suggest-ArticleSignals.ps1: suggestion engine hardened with a stable JSON
  contract (schemaVersion/toolVersion) and a deterministic, effect-sensitive
  per-proposal suggestionId, plus -ChangedFiles scoping for PR-diff runs. Runs
  self-contained: the routing seed is now OPTIONAL (built-in domain map), so this
  does not depend on the separate routing-index tuning thread.
- tools/New-AuthoringAssistComment.ps1: renders one upsertable, feedback-instrumented
  advisory comment (stable anchor + aa:meta/aa:article markers carrying suggestionIds
  + reaction footer) for the downstream acceptance-measurement work.
- .github/workflows/authoring-assist*.yml: unprivileged pull_request intake +
  trusted workflow_run runner (review/publish split) + in-repo self-test.
- tools/Test-SuggestArticleSignals.ps1: 23-check smoke test (contract, determinism,
  scoping, suppressor/prohibition, renderer markers, seed-free graceful degradation).

Dormant schema support:
- .github/scripts/validate_frontmatter.py: adds R29, validating the optional
  `signals` block shape (bare token or mapping with token + optional
  pattern/domain/effect in {raise,suppress}). Reserved & dormant — no production
  pipeline consumes it; the existing PR Reviewer only reads `effect: suppress`
  behind its BCQ_INDEX_V2 flag and falls back safely when absent. Existing rules
  (incl. R24 skill-id uniqueness, R27, R28) are unchanged.

Excludes the routing-index tuning thread entirely (separate work).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 75d852d6-18a6-4c58-986f-ed5ab16618fa

.github/workflows/authoring-assist-selftest.yml

@@ -0,0 +1,33 @@
+name: Authoring-assist self-test
+
+# Regression-guards the authoring-assist feature (suggestion engine + JSON
+# contract + comment renderer) against this repo's own corpus. Runs on changes
+# to the tool, renderer, the optional routing seed, or the test itself.
+
+on:
+  pull_request:
+    branches: [main]
+    paths:
+      - 'tools/Suggest-ArticleSignals.ps1'
+      - 'tools/New-AuthoringAssistComment.ps1'
+      - 'tools/Test-SuggestArticleSignals.ps1'
+      - 'tools/routing-seed.json'
+      - '.github/workflows/authoring-assist*.yml'
+  push:
+    branches: [main]
+    paths:
+      - 'tools/Suggest-ArticleSignals.ps1'
+      - 'tools/New-AuthoringAssistComment.ps1'
+      - 'tools/Test-SuggestArticleSignals.ps1'
+      - 'tools/routing-seed.json'
+
+jobs:
+  selftest:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Run authoring-assist smoke test
+        shell: pwsh
+        run: ./tools/Test-SuggestArticleSignals.ps1 -BCQualityRoot .