feat: close Muse ACP sessions cleanly

[obviyus]

Problem

The adapter does not advertise or implement ACP session/close. ACPX and other clients cannot ask it to cancel active work and release adapter session resources through the protocol.

Fix

• Advertise sessionCapabilities.close and register session/close.
• Reserve each turn with a completion promise from prompt compilation through final cleanup.
• Revoke new prompt admission synchronously when close starts.
• Cancel the active Muse process, wait for prompt cleanup, then acknowledge close.
• Preserve Muse's native session log so session/list and session/load recovery continue to work.
• Keep unknown and already-closed session errors explicit.

Lifecycle proof

• Idle close releases the adapter session and later prompts fail visibly.
• Mid-turn close reaps the Muse child and removes staged images before returning.
• Close during image staging prevents Muse from spawning.
• A prompt cannot enter while close waits.
• A concurrent second close receives the consistent unknown-session error.
• Close still returns when the active prompt rejects during unwind.
• A real Muse echo session remains in the native log and session/list after close.

Verification

• npm run check
• npm run build
• npm run test:run — 116 passed, 2 live-provider tests skipped
• Focused session-close tests — 6 passed
• P1 autoreview — clean
• Fable lifecycle design and implementation reviews — clean

Dependencies

This draft is stacked on #2 and #3. Merge those first; GitHub will then reduce this PR to the close lifecycle change.