Add mobile VISION (#7710)

## Summary

Add the Buzz Mobile vision covering intended experience, architecture,
shared client behavior, verification, release cadence, and critical
functionality. Link it from the mobile README; the existing agent
instruction to read relevant `VISION_*.md` documents makes it
discoverable during planning and review.

### Related issue

No duplicate issue or PR found.

### Testing

Documentation-only change. Diff checks and pre-push hooks passed.
Repository-wide `just ci` is running locally; Rust formatting has
passed.

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Signed-off-by: Tom Brow <tomb@block.xyz>

AGENTS.md

@@ -112,6 +112,10 @@ See CONTRIBUTING.md for full setup details and dependency requirements.
 Run `just ci` before every PR — it runs repository-wide formatting, lint,
 and static checks; Rust, Tauri, desktop, and mobile tests; and desktop and web
 builds. Clippy passing does not mean fmt passes; run both.
+For changes limited to Flutter/Dart code in the mobile app, run
+`just mobile-install mobile-check mobile-test` instead of `just ci`.
+Native code and build-configuration changes also require the corresponding
+platform checks.
 
 Run `just test` for integration tests if you touched `buzz-relay`,
 `buzz-db`, or `buzz-auth` — these require a running Postgres and Redis.