docs: add cross-context feature docs for auth, presence, access-control, messaging, attachments

Fills the five highest-value gaps under agents-docs/features/ so the index covers
the system's main cross-context contracts. Each doc follows the feature-template
structure and the AGENTS_FEATURES.md contract, with honest TODOs where coverage
or behavior couldn't be confirmed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agents-docs/FEATURES.md

@@ -8,8 +8,13 @@ It must stay accurate as new features are introduced, renamed, merged, or remove
 
 ## Feature list (alphabetical)
 
+- [access-control](./features/access-control.md) — Roles, role assignments, channel permission overrides, memberships, invites, bans, and slowmode.
+- [attachments](./features/attachments.md) — P2P chunked file-transfer protocol over the WebRTC chat data channel, storage decisions, auto-download rules.
+- [authentication](./features/authentication.md) — User account REST surface, WebSocket `identify` handshake, heartbeat sweep, and Electron Local API tokens.
 - [ipc-bridge](./features/ipc-bridge.md) — Electron preload `window.electronAPI` surface, IPC channels, and CQRS dispatch.
+- [messaging](./features/messaging.md) — Server-channel chat, direct messages, inventory-sync protocol, delivery state machine.
 - [plugin-system](./features/plugin-system.md) — Plugin manifest contract, renderer runtime, capability grants, and server `plugin-support` API.
+- [presence](./features/presence.md) — Connection lifecycle, availability status, profile metadata propagation, voice membership, and game activity.
 - [server-directory](./features/server-directory.md) — REST surface for server catalog, invites, join requests, and moderation.
 - [voice-signaling](./features/voice-signaling.md) — WebRTC mesh signaling, RNNoise pipeline, and voice / direct-call / screen-share orchestration.
 - [websocket-envelopes](./features/websocket-envelopes.md) — Wire-format contract for every realtime envelope between server and clients.