docs(test): finish Claude Desktop 3P guides and end-to-end acceptance after #349 #361

The implementation portion of draft PR #351 has been integrated into parent PR #349 and is now merged into main at 52bc938f2c5eddda5cee44f16c4f542009d034b9. Its real concurrency, installer, doctor and sync-review findings have been addressed with exact-head green CI and resolved review threads. Do not re-merge the stale child branch.

The original #351 checklist included additional deliverables not yet part of the integrated patch:

• User-facing docs-site guide, CLI reference and troubleshooting for Claude Desktop 3P setup, sync and doctor, including Free-account/vendor-gated capability limitations; keep documented real vs synthetic integration evidence distinct.
• CHANGELOG and short ADR explaining cross-process library ownership, async HTTP locks, automatic vs manual sync semantics and safe uninstall.
• Evaluate bin/ocx.mjs optional orphaned-protocol repair hint after update; add only if there is a proven owner-safe contract and tests (never auto-overwrite foreign Desktop entries).
• Extend regression tests for concurrent lock contention, stale-token reclamation, orphan/association cleanup, discovery-only/hybrid/static profiles and failed-Unlink behavior. The latest fix(claude-desktop): clean uninstall, private backups, shared library lock, doctor #349 regressions cover automatic under-lock merge, discovery-only profile drift and unknown Linux handler registration, but not all integration variants.
• Live browser/installed Claude Desktop 3P acceptation per parent P0 issue P0: OCX-only first (Claude Code, Droid, Codex, Claude Desktop) + optional native, no regressions #336, without claiming vendor-gated features unlocked.

Acceptance: documented supported/unsupported modes align with the actual CLI and API, security/privacy boundaries hold, exact-head CI passes, and no global session restart or native login overwrite. Reuse #336's acceptance plan and avoid duplicate backend abstractions.

References: parent PR #349, superseded draft #351, P0 clients #336.