ACP: /session delete, cleanup and prune fail with "sessionsApi ... is unavailable for this session" #5268

Describe the bug

In ACP mode (copilot --acp --stdio), Copilot advertises /session in available_commands_update, but its delete, cleanup and prune subcommands fail over ACP. Sent as a session/prompt, each ends with end_turn in about 1 ms, without a model call, and replies with an error:

Nothing is deleted: the sessions' folders stay in session-state.

An ACP client has no other way to delete Copilot sessions. initialize offers sessionCapabilities list and close only, without ACP's stable session/delete.

Affected version

GitHub Copilot CLI 1.0.104

Steps to reproduce the behavior

1. Start copilot --acp --stdio and initialize.
2. Call session/new twice, for sessions A and B.
3. Send session A a session/prompt with the text /session delete <B's sessionId>.
4. The reply is the sessionsApi.list error above, with stopReason: end_turn. Session B's folder is still in session-state.
5. Send /session cleanup, then /session prune --older-than 1 --dry-run --include-named. Each gets its error above.

Expected behavior

These subcommands work over ACP as they do in the interactive CLI:

• /session delete <id> deletes the session.
• /session cleanup removes empty or abandoned sessions.
• /session prune --older-than <days> deletes old sessions, or lists them with --dry-run.

Each replies with what it did.

Better still for ACP clients: advertise sessionCapabilities.delete and support session/delete. Then a client can delete a session without sending a prompt through another one.

Additional context

• Windows 11, x86_64, driven from a Node ACP client, in a scratch COPILOT_HOME.
• prune checks its arguments before it fails. Without --older-than it replies with its usage, and --older-than 0 gets Invalid number of days: 0. It fails once it needs the sessions API.
• This is like ACP: /rename reports success but doesn't rename the session #5115. There, /rename over ACP replies that it renamed the session but saves nothing. In both cases the command is advertised over ACP, but the ACP session lacks what it needs.
• Related: Support all slash commands in ACP #2555 (advertising every slash command over ACP) and /cleanup or command to remove empty/abandoned sessions #1451 (/cleanup).