feat(core): auto-background slow Code Mode executions - #60709

Code Mode execute calls now yield as a single background job after 10 seconds while keeping nested tool calls typed in foreground inside the script.
Instead of rewriting inner tools.* return types for backgrounding, execute itself runs through executeTopLevel, streams live console.log lines and nested call status into partial progress, delivers an <execute> notice when done, and exposes "tool" jobs to idle hooks and goals.

💡 Why

• Auto-backgrounding inner tools.* calls inside execute would break every Code Mode script expecting typed return values.
• Active goals and session idle hooks need to see running "tool" jobs so a goal waits for a backgrounded Code Mode or MCP call before judging the round.

🆕 Now you can

• Let slow execute scripts auto-background after 10 s (or via yield_after / background: true), returning console.log output and nested call progress so far and delivering <execute id="..." state="completed"> even after location invalidation (shell: plugin suffix rebuild drops background completion notifications #56729).
• Inspect running "tool" jobs in SessionIdleBackground and hold GoalPlugin.idle until they finish.

- if (name === "execute" && codeModeTool) return yield* executeTool(codeModeTool, ...)
+ if (name === "execute" && codeModeTool) return yield* executeTopLevel(codeModeTool, ...)
+ SessionIdleBackground |= { kind: "tool", sessionID, tool, title? }
+ GoalPlugin.idle waits when background has kind === "tool" for sessionID

📊 Numbers

New Code Mode auto-background test in tool-auto-background.test.ts fails without this change; 146 tests pass across codemode.test.ts, tool-auto-background.test.ts, job.test.ts, tool-image-generate.test.ts, session-execution.test.ts, and plugin/goal.test.ts.

👀 Read: packages/core/src/tool.ts (executeTopLevel name === "execute" branch) and packages/core/src/plugin/goal.ts (idle waiting on work.kind === "tool"). The rest is routine.

_{🔍 Split 6 of 6 from #60495 · closes out the #60495 split}