acp: Support session modes (e.g. CC plan mode) (#37632)

Adds support for [ACP session
modes](https://github.com/zed-industries/agent-client-protocol/pull/67)
enabling plan and other permission modes in CC:


https://github.com/user-attachments/assets/dea18d82-4da6-465e-983b-02b77c6dcf15


Release Notes:

- Claude Code: Add support for plan mode, and all other permission modes

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

assets/settings/default.json

@@ -828,6 +828,9 @@
       // }
     ],
     // When enabled, the agent can run potentially destructive actions without asking for your confirmation.
+    //
+    // Note: This setting has no effect on external agents that support permission modes, such as Claude Code.
+    //       You can set `agent_servers.claude.default_mode` to `bypassPermissions` to skip all permission requests.
     "always_allow_tool_actions": false,
     // When enabled, the agent will stream edits.
     "stream_edits": false,