fix(agent): a racing cancel can no longer wedge the session's turn slot

Port of upstream grok-build 0.2.103. session/prompt runs an async intake
preamble before enqueueing SessionCommand::Prompt on the actor mailbox; a
concurrent cancel could enqueue Cancel first, land on an idle actor as a
no-op, and the prompt then started a turn nothing would ever cancel.

The per-session prompt-intake lock becomes the dispatch lock: prompt()
holds it across intake as before, and cancel() now holds it around its
Cancel send, so a cancel orders behind the prompt it targets. Regression
test ported verbatim (unix-only harness, like its siblings).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-shell/src/agent/mvp_agent/agent_ops.rs

@@ -978,7 +978,7 @@ impl MvpAgent {
             sessions: RefCell::new(HashMap::new()),
             activity,
             loading_sessions: RefCell::new(HashMap::new()),
-            prompt_intake_locks: RefCell::new(HashMap::new()),
+            dispatch_locks: RefCell::new(HashMap::new()),
             session_threads: RefCell::new(HashMap::new()),
             resident_roster_titles: RefCell::new(HashMap::new()),
             initialize_request: OnceLock::new(),