docs: populate initial cross-context feature docs

Add area-level documentation for the five most significant cross-context
feature areas under agents-docs/features/:

- websocket-envelopes: full envelope catalogue, lifecycle, dispatcher
- ipc-bridge: window.electronAPI surface, IPC channels, CQRS dispatch
- plugin-system: manifest contract, runtime, capabilities, plugin-support API
- server-directory: REST endpoints, CQRS, entities, business rules
- voice-signaling: mesh signaling, RNNoise pipeline, domain split

Update agents-docs/FEATURES.md index alphabetically and remove the
"no cross-context feature docs" placeholder.

Each doc records honest TODOs for verified gaps (stale signaling-contracts.ts,
window.api vs window.electronAPI mismatch, IPC error envelope drift from
CONTEXT.md, missing OpenAPI coverage for server-directory routes, no
envelope round-trip test).

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

agents-docs/FEATURES.md

@@ -8,7 +8,11 @@ It must stay accurate as new features are introduced, renamed, merged, or remove
 
 ## Feature list (alphabetical)
 
-_No cross-context feature docs have been written yet._
+- [ipc-bridge](./features/ipc-bridge.md) — Electron preload `window.electronAPI` surface, IPC channels, and CQRS dispatch.
+- [plugin-system](./features/plugin-system.md) — Plugin manifest contract, renderer runtime, capability grants, and server `plugin-support` API.
+- [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.
 
 The product client already documents its bounded contexts at `toju-app/src/app/domains/<name>/README.md` (Access Control, Attachment, Authentication, Chat, Direct Call, Direct Message, Experimental Media, Game Activity, Notifications, Plugins, Profile Avatar, Screen Share, Server Directory, Theme, Voice Connection, Voice Session). Those domain READMEs cover internal product-client behavior.