fix: add waiting status so agents can end turns without continuation spam

[hieusats]

Problem

While a goal is active, agent_end unconditionally queues a continuation turn. When the only correct action is to end the turn and passively wait for an external wake — exactly what pi advises for background task notifications — this creates an infinite no-op loop:

agent: "Worker still running — waiting for notification, not polling."
agent_end -> continuation queued -> new turn -> same reply -> agent_end -> ...

Observed in a real session: 383 continuation events at ~5 second intervals, each one a paid API call, for as long as the background task ran. The agent behaved correctly (no polling); the extension punished that behavior. Worse, because the wake notification arrives as a pending message and then the loop resumes, every external event adds another no-op cycle.

There is currently no way for the model to declare "I am blocked on an external event — do not re-trigger me".

Fix

• New waiting goal status, set by the model via update_goal({ status: "waiting" })
• agent_end does not queue a continuation while waiting
• Any externally triggered turn (user message or event notification) auto-resumes the goal to active at turn_start, so normal continuation resumes after the wake
• The continuation prompt teaches the model to mark waiting instead of replying with no-op status messages
• statusLine / event labels cover the new status; README documents it

Contract note

waiting is a scheduling hint, not lifecycle control: pause, resume, clear, and budget limiting remain user/runtime-owned. The source-contract test is updated to pin the new schema (enum: ["complete", "waiting"]) while keeping the "no pause/resume/abandon/budget-limit via update_goal" assertion. Usage is still accounted on waiting turns, and budget_limited still flips on the next active turn_end.

Tests

• New test/agent-loop.test.cjs: runtime harness driving the real extension code through jiti with mocked ExtensionAPI/ExtensionContext — asserts exactly-one continuation while active, zero continuation while waiting, auto-resume on the next turn, continuation resuming after the wake, idempotent waiting, rejection of lifecycle statuses, and usage accounting during waiting
• test/goal-state.test.cjs: statusLine + goalEventStatus cases for waiting
• test/source-contract.test.cjs: updated update_goal schema contract
• npm test → 28/28 pass; npm run check passes