feat: wire channels through pipeline, remove dead code, add metadata version

[ronaldtse]

Summary

Completes the channel-based publication wiring from the audit (TODO 15, items A–K):

What changed

1. Channel pipeline wiring (audit A, F, G)

   • Replaced VisibilityFilter with ChannelManifestFilter that uses ChannelManifest.resolve() for policy-based filtering
   • Pass manifest to pipeline deps so resolveChannels() and passesStageConstraint() route per-document channels to the publisher

2. Dead code removal (audit B, C, D, E)

   • Removed ReleaseManifest class (superseded by ChannelManifest)
   • Removed Visibility enum (no production references)
   • Removed VisibilityFilter (replaced by ChannelManifestFilter)
   • Removed isPublic() and resolveStageAllowList() bridge methods from ChannelManifest
   • All tests updated to use resolve() instead of isPublic()

3. Metadata & robustness (audit H, I)

   • Added version: 1 to ReleaseMetadataJson for format negotiation by downstream consumers
   • Added warning log when resolveSourcePath() cannot find a /documents/ segment

4. O(n²) fix (audit J) — already in place from prior work, verified no indexOf in pipeline

Test plan

• 298 tests pass (25 files)
• New channel-manifest-filter.test.ts with 5 tests covering the filter
• Updated pipeline.test.ts, channel-manifest.test.ts, manifest-loader.test.ts to use new APIs
• Removed release-manifest.test.ts and visibility-filter.test.ts
• Build, lint, format all pass