Updates documentation to match current codebase implementation:
- Deprecates `nostrClient.fetchVideos` in README.md, pointing to `subscribeVideos`.
- Clarifies `playVideoWithFallback` implementation location in `docs/playback-fallback.md` and `AGENTS.md` (moved to `js/app/playbackCoordinator.js`).
- Corrects `WATCH_HISTORY` d-tag description in `docs/nostr-event-schemas.md` to match actual code behavior (`<YYYY-MM>`).
Co-authored-by: PR0M3TH3AN <[email protected]>
