chore(docs): widen docs-consistency test, archive stale concept docs

The TestDocsConsistency walk only iterated [".", "guides", "reference",
"analysis"] — concepts/ was silently invisible, which is why
amazon-music-oauth.md slipped into the tree without a SUMMARY entry.

Refactored to walk the entire docs/ tree, with a small dirsToSkip
allow-list (_includes, archive, diagrams, images) for asset trees.
New top-level narrative directories are picked up automatically;
only asset dirs need an explicit entry.

The wider walk surfaced six previously-hidden concepts/* files. Five
older planning artefacts ("Enhanced State Management System",
"Upstream Bose Service Simulation") moved into docs/archive/ where
the dirsToSkip already excludes them; concepts/README.md renamed to
upstream-service-simulation-overview.md since "README.md" inside
archive/ would be misleading. Spotify Overview and Amazon Music
OAuth are user-facing narrative docs and are now linked under
Concepts in SUMMARY.md.

Note: concepts/streborn-patterns.md is internal review notes (its
own opening line says so) and is currently unlinked from SUMMARY.md;
will be handled separately by the maintainer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/SUMMARY.md

@@ -53,8 +53,10 @@
 
 ## Concepts
 * [Request Recording](REQUEST_RECORDING_CONCEPT.md)
+* [Spotify Overview](concepts/spotify-overview.md)
 * [Spotify Priming Strategy](concepts/spotify-priming-strategy.md)
 * [Spotify OAuth](concepts/spotify-oauth.md)
+* [Amazon Music OAuth](concepts/amazon-music-oauth.md)
 * [Encrypted Export](concepts/ENCRYPTED-EXPORT.md)
 * [Diagnostic Export (Maintainer Setup)](DIAGNOSTIC-EXPORT.md)
 * [soundtouch-web Roadmap](soundtouch-web-roadmap.md)