Bound and cancel TUI AI suggestion work

[jmqd]

AI suggestions spawn provider work for focus changes, but cancellation drops local state while provider calls keep running. Fast navigation can accumulate background work and degrade TUI responsiveness. Use a bounded worker, debounce, generation checks, or cooperative cancellation.

[jmqd]

Context: TUI audit found focus changes can spawn provider work repeatedly. The existing cancel path drops local suggestion state, but in-flight provider calls can continue. No local fix in this turn; tracked as UX responsiveness risk.

[jmqd]

Patch: 19630a1 handles AI suggestion worker spawn failure as a UI error instead of panicking the TUI when the OS refuses a new thread. Verification: cargo test ai_worker_spawn_failure_reports_error --lib; cargo test ai_suggestion --lib; just check passed with 986 tests, clippy, and fmt. Remaining issue scope: stale provider processes still need bounded/cancelled work.