refactor(core): deliver image generation notices via the job service - #60708

Image generation now delegates completion notices to the job service, deleting 32 net lines from image-generate.ts.
Passing sessionID to jobs.start lets Job format and deliver <image> completion, failure, and cancellation notices directly, removing ImageGenerateTool.notifyWhenDone and its plugin-scoped watcher fiber.

❌ Before

• ImageGenerateTool pre-allocated notificationID, forked notifyWhenDone in the plugin scope, and formatted ImageNotice itself.

✅ After

• ImageGenerateTool passes sessionID: context.sessionID to jobs.start and calls jobs.background(id); Job handles persistence, formatting, delivery, and cleanup.

🟰 Behavior change: notices survive plugin/location reloads instead of depending on the plugin's scope; output format and metadata are unchanged.

- const notifyWhenDone = Effect.fn("ImageGenerateTool.notifyWhenDone")(...)
- yield* jobs.start({ id, notificationID, ... })
- yield* notifyWhenDone({ sessionID, id, notificationID, file: stem })
+ yield* jobs.start({ id, sessionID: context.sessionID, ... })
+ yield* jobs.background(id)

📊 Numbers

Tests: all 15 tests in tool-image-generate.test.ts pass.

👀 Read: packages/core/src/tool/plugin/image-generate.ts around jobs.start. Everything else is deletion.

_{🔍 Split 5 of 6 from #60495 · shrinks production code by 32 lines}