feat(core): auto-background slow tools after 10s through Tool.execute - #60495

Slow shell commands, Code Mode scripts, MCP calls, and custom plugin tools now move to the background automatically after 10 seconds instead of blocking the turn or getting killed at 2 minutes.
Fast calls (< 10 seconds) and instant local mutators or prompts (edit, write, patch, question, todowrite, read, grep, glob) behave identically to before. Targets dev directly (#60491 is merged).

🔍 Origin: reported in #50278, #54221, #49216, and #56729

💡 Why: Previously only shell, subagent, and experimental_image_generate hand-rolled background jobs — each forking its own plugin-scoped notifyWhenDone watcher that dropped completions on plugin reload (#56729) — while execute (Code Mode), MCP tools, and custom plugins blocked the turn with no way to background them (#54221, #49216).

🆕 Now you can: Run long builds, servers, Code Mode scripts, or slow MCP/plugin calls and have them automatically yield status: "running" with a jobID and live output/logs after 10 seconds (yield_after overrides per call; 0 or background: true backgrounds immediately), or move any active tool to the background with Ctrl+B.

flowchart LR
  Call["Top-level tool call<br/>(shell · execute · MCP · plugins)"] --> Seam["Tool.executeTopLevel + Job.block (10s)"]
  Seam -->|"finishes < 10s"| Inline["Return inline output"]
  Seam -->|"10s yield_after / Ctrl+B"| Bg["Return jobID + output so far"]
  Bg --> Settle["Global Job.settle -> Session.synthetic wake"]
  style Seam fill:#dcfce7,stroke:#16a34a,color:#14532d
  style Settle fill:#dcfce7,stroke:#16a34a,color:#14532d

- ShellTool / ImageGenerateTool: bespoke plugin-scoped notifyWhenDone + duplicate restart formatters
- execute (Code Mode) / MCP / plugin tools: block until completion; Ctrl+B is a no-op
+ Tool.executeTopLevel + Job.block({ yield_after: 10_000 }): wraps execute, MCP, web, and plugin tools automatically
+ Global Job.settle + Job.formatNotice: single completion & restart delivery path (survives Location reload)
+ Tool.Options.background = false: instant mutators & prompts (edit, write, patch, question) stay synchronous

📊 Proof

New integration tests in packages/core/test/tool-auto-background.test.ts fail without the change; 136 tests across 6 codemode and core suites pass.

👀 Read: packages/core/src/tool.ts in executeTopLevel and packages/core/src/job.ts in block — executeTopLevel wraps only top-level Snapshot.execute calls so nested tools.* calls inside Code Mode keep their declared JS return types, and block resolves foreground vs background ownership under the state.jobs lock so boundary completions never double-deliver. The rest is routine.