feat(core): deliver background job notices from the job service - #60705

Background job completion notices now deliver from the global job service instead of plugin-scoped fibers.
Previously each tool forked its own notifyWhenDone fiber inside a location-scoped plugin, so reloading or closing a location mid-run could orphan notice delivery. Centralizing delivery in Job keeps background runs alive until the notice reaches the session inbox.

💡 Why

• Plugin scopes close when a project location reloads or invalidates (shell: plugin suffix rebuild drops background completion notifications #56729), cutting off per-plugin notification watchers while the underlying job is still running.
• Tools that return structured values alongside notice text had to maintain separate Deferred plumbing beside Job.start.

🆕 Now you can

• Pass sessionID, output (live partial output reader), stop, and { output, value } to Job.start, with Session registering a single global notifier via Job.registerNotifier.
• Inspect live or settled jobs through Job.read and Job.list, and cancel silently with Job.cancel(id, { notify: false }).

- plugin scope forks notifyWhenDone -> jobs.wait -> sessions.synthetic -> completeBackground
+ Session registers notifier once on Job.Service
+ Job.settle / markBackground / cancel deliver notice in global scope -> completeBackground

📊 Numbers

Tests: 53 pass across job.test.ts and session-execution.test.ts.

👀 Read: packages/core/src/job.ts around deliverNotice and autoNotify: auto-delivery activates only when callers opt in with sessionID on Job.start, keeping existing callers (ShellTool, ImageGenerateTool) duplicate-free until their follow-up PRs drop notifyWhenDone. The rest is routine.

_{🔍 Split 2 of 6 from #60495 · stacked on #60704}