feat(maestro): add acp_session_id to the session sidecar (#134)

A Structured session's adapter hands back an ACP session id from
session/new; a respawn will pass it to session/load (#135). This is a
new field rather than #161's agent_session_id: that one is the Claude
Code conversation id the activity hook reports and `claude --resume`
takes, minted by Claude Code itself. The ACP id is minted by the
adapter, only exists for Structured sessions, and is what the adapter
takes back. Overloading one field with two ids from two processes
would leave a respawn unable to tell which kind it holds.

web/src/types.ts

@@ -56,6 +56,10 @@ export interface Session {
    * how the command bar's empty query sorts a never-focused session after every
    * session that has one (`docs/COMMAND-BAR.md`). */
   last_focused_at?: IsoTimestamp | null;
+  /** The ACP session id the `claude-code-acp` adapter returned from
+   * `session/new` (#134). Only a `Structured` session whose handshake
+   * succeeded has one; a respawn passes it to `session/load` (#135). */
+  acp_session_id?: string | null;
 }
 
 /** A session's daemon-created worktree (`SessionWorktree` on the wire). */