[Feature Request] ACP: support _cognition.ai/session/rename so session renames in Devin Desktop persist

Summary

kimi-code's ACP server does not implement the _cognition.ai/session/rename extension method, so renaming a kimi session in Devin Desktop's sidebar never persists — the UI rename is silently lost, and the session keeps showing its auto-derived title (the raw first user message, e.g. a long <resource uri="..."> blob).

Reproduction

1. Run kimi-code as an ACP agent in Devin Desktop (v3.6.27, Linux; both Local and Remote-SSH setups).
2. Create a kimi session, then rename it in the sidebar.
3. The rename never reaches the kimi session store: state.json keeps "isCustomTitle": false and the old auto title.

Protocol-level evidence

Sending the extension method directly to kimi acp:

>>> {"jsonrpc":"2.0","id":2,"method":"_cognition.ai/session/rename",
     "params":{"sessionId":"session_58fe…","title":"probe"}}
<<< {"jsonrpc":"2.0","id":2,"error":{"code":-32601,
     "message":"\"Method not found\": _cognition.ai/session/rename", ...}}

availableCommands in the session/new response is also empty, so /title (the TUI rename command) is not reachable via ACP either.

Why this should be a small change

The rename machinery already exists:

• /title (alias /rename) slash command persists custom_title into state.json (see #1536, PR #1543, PR #1685)
• renameSession in the web/harness API
• state.json already carries title + isCustomTitle fields

The ACP layer just needs to route _cognition.ai/session/rename ({sessionId, title}) to that existing path (set custom title + isCustomTitle=true so auto-title never overwrites it).

If a standardized ACP rename method is adopted later, supporting that instead would equally resolve this.

Environment

• kimi-code 0.34.0 (linux x64, native binary)
• Devin Desktop 3.6.27 (Linux), ACP protocolVersion 1

Start at the kimi acp entry point and inspect how ACP methods are routed, then compare it with the existing /title or /rename command and the renameSession web/harness API. Verify the request using _cognition.ai/session/rename and confirm that state.json persists the requested title with isCustomTitle set to true.