refactor(core): share job notice formatting and shell output snapshots - #60704

Share background job notice formatting and live shell output snapshots ahead of auto-backgrounding.
Startup recovery and live background tools format the same shell and image completion notices, and shell result capture only worked after exit. Extracting both helpers keeps behavior identical today while preparing live job reads and central notice delivery.

❌ Before

• session/execution/restart.ts owned a private restartMessage formatter for shell and image background notices.
• Shell.result computed bounded output tails inline only after wait completed, so callers could not read bounded partial output from a still-running command.

✅ After

• Job.formatNotice formats shell and image notices in one place, and restart.ts calls it with { restarted: true }.
• Shell.snapshot reads the bounded output tail without waiting for exit, and Shell.result reuses it.

🟰 Behavior change: none.

- Shell.result -> wait(id) -> inline tail slicing
- restart.ts -> private restartMessage(background, recovery)
+ Shell.snapshot(id) -> bounded tail anytime; Shell.result reuses it
+ Job.formatNotice(background, recovery, { restarted: true })

📊 Numbers

Tests: 41 existing tests pass across job.test.ts and session-execution.test.ts.

👀 Read: packages/core/src/shell.ts around snapshot, ensuring empty running output returns "" while completed empty output still renders "(no output)". The rest is routine.

_{🔍 Split 1 of 6 from #60495 · no user reports}