feat(core): yield slow top-level tools to the background after 10s - #60706

Slow top-level plugin and MCP tools now move to the background after 10 seconds instead of blocking the turn.
Long tool calls no longer stall the agent loop: after yield_after (10 s by default, or Ctrl+B), Tool.execute returns partial progress immediately and delivers the bounded result later as a <tool> notice. Fast local file, search, and interactive tools opt out with background: false.

💡 Why

• Slow MCP calls and custom plugin tools blocked the entire turn with no way to yield or steer past them.
• Interactive prompts (permissions and forms) must pause the yield timer so waiting on a person never backgrounds a tool mid-approval.

🆕 Now you can

• Let any top-level tool without background: false auto-background after 10 s (or on Ctrl+B), preserving ToolOutput truncation and saving returned file attachments for background notices.
• Pause yield_after automatically whenever the session has a pending permission or form prompt.

- Tool.execute -> executeTool(tool, name, input, context) (blocks until exit)
+ Tool.execute -> executeTopLevel -> jobs.start + jobs.block({ yield_after: 10_000, deferYield })
+   on yield -> return partial progress + jobID; deliver bounded <tool> notice on completion

📊 Numbers

New tests in tool-auto-background.test.ts fail without this change; 58 tests pass across job.test.ts, tool-auto-background.test.ts, and session-execution.test.ts.

👀 Read: packages/core/src/job.ts (block resolving isBackgrounded vs consumeJob atomically in one SynchronizedRef.modify) and packages/core/src/tool.ts (executeTopLevel + renderResultText). The rest is routine (background: false one-liners on synchronous built-ins).

_{🔍 Split 3 of 6 from #60495 · no user reports}