feat: add pi-goal agent mode

[AhmedKhattab1993]

Closes #161.

Companion provider PR: narumiruna/pi-extensions#298

Summary

• add first-class Agent({ goal: true }) schema, manager, runner, and documentation support
• give Goal-mode children a private session event bus, start pi-goal after extension binding, and wait for an authoritative terminal state plus session idle before cleanup
• pause goals safely on parent abort or hard turn limits, including cancellation before the start reply
• preserve worktree/background behavior while failing fast for missing/excluded pi-goal, removed Goal tools, isolated: true, resume, and schedule
• report only complete as success; blocked/paused/limited outcomes retain their Goal result through the failed/stopped lifecycle

Verification

• npm run lint
• npm run typecheck
• npm run build
• hermetic full suite: 752 passed, 5 skipped
• E2E suite: 44 passed, 5 skipped
• real-loader Goal protocol E2E: passed
• cross-repository smoke with the actual pi-goal source, real Pi loader/session, and goal_complete: passed