[docs](trx-rs): remove appkit frontend references

Remove AppKit frontend mentions from documentation:
- Update AGENTS.md project structure
- Remove AppKit from capabilities table in OVERVIEW.md
- Remove AppKit from frontends table in OVERVIEW.md
- Remove AppKit from Frontends section in README.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

README.md

@@ -17,7 +17,6 @@ The rig task is now driven by the controller components (state machine, handlers
 
 - HTTP status/control frontend (`trx-frontend-http`)
 - JSON TCP control frontend (`trx-frontend-http-json`)
-- AppKit GUI frontend (`trx-frontend-appkit`, macOS only, optional via `appkit-frontend` feature)
 - rigctl-compatible TCP frontend (`trx-frontend-rigctl`, listens on 127.0.0.1:4532)
 
 ## Audio streaming