Run the integration harness from its own devshell so the server packages stay out of the everyday shell. Add lms and a Nextcloud Music instance to the harness and extend the suite with auth-fallback, media, error, write round-trip, multi-source and prune tests.
The auth test caught that lms answers OpenSubsonic error 42 (not 41) when it refuses token auth, so the client now falls back on both codes.
Spell out the sync contract: browsing, filtering and sorting never touch the network,
only media bytes and images are on-demand, and a server-derived sort key has to land in a
local column at sync time rather than being requested lazily.
adds docs/architecture.md (stack, layers, data model, sync, search, networking, testing, CI), docs/implementation-plan.md (phases and status), and AGENTS.md for agents and contributors