fix: observe owned background terminals between turns

[naiba]

Problem

Background terminal reconciliation currently depends on turn/item events. Once the parent is idle, a shell can disappear from the native terminal list without its ACP task being reconciled.

Change

Keep one unref-ed, serialized observation timer for known tasks and their owning threads. Respect app-server replacement/disposal fences; stop polling when tasks disappear. This does not start model turns or inject completion prompts.

Validation

• 32 async-task tests passed, including idle reconciliation, no overlapping requests, disposal, rejected observation retry, and cessation after removal.
• Typecheck and build passed.
• Full local Bun suite passed without retries: 1493 passed, 36 pre-existing skipped (before the additional idle-removal regression, which passed separately). Full-suite run also applied separate fixture-cleanup and workspace-symlink test fixes; neither is included in this PR.
• A previous Node full-suite run failed in existing deep JSON serialization and native initialization tests; these are not represented as passing.