Own knowledge-index generation in BCQuality (runtime + CI), not the consumer

The index is now produced by BCQuality itself: Entry's preparation step
rebuilds knowledge-index.json over the live, already-pruned clone at the start
of every run, and a new CI workflow validates the generator's health
(determinism, full coverage, selection-input integrity). Consumers no longer
invoke or know about the index.

Rebuilding over the pruned clone (vs shipping a committed full-corpus index)
keeps the index exact for any consumer policy: it can never list a denied
article, so policy-excluded rules cannot leak into discovery. READ now states
the index is discovery-only -- a finding must cite an article opened in full,
and rows whose file is absent are discarded before ranking.

- skills/entry.md: new 'Preparation -- knowledge index' precondition
- skills/read.md: index ownership + discovery-only invariant
- microsoft/skills/review/*.md (6): 'BCQuality builds' (not 'the filter emits')
- agent-consumption.md 5a: runtime+CI ownership rationale
- .github/workflows/knowledge-index.yml + scripts/Test-KnowledgeIndex.ps1: generator guard
- .gitignore: never commit the runtime index

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

.github/workflows/knowledge-index.yml

@@ -0,0 +1,24 @@
+name: Validate knowledge index
+
+# BCQuality owns the knowledge-index generator (tools/Build-KnowledgeIndex.ps1),
+# which the AL review skills' Source step consumes. This guards the generator's
+# health so consumers never have to: the index a consumer uses at runtime is
+# rebuilt over its own pruned clone, but the generator that builds it lives and
+# is validated here.
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  validate-index:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Validate knowledge-index generator
+        shell: pwsh
+        run: ./.github/scripts/Test-KnowledgeIndex.ps1 -Root .