feat: add token-free Pi Hunk grid command

[almogdepaz]

summary

• package a token-free Pi /hunk command that creates a Wolfpack shell beside the current session and starts exec hunk diff --watch
• extend canonical session creation with explicit shell and --grid parent context while keeping OpenableHarness agent-only
• pin and recheck stable parent identity before the existing sub_session_opened notification; preserve typed partial-session details on stale-parent failures
• validate structured CLI responses, use fixed argv, bound command duration/notifications, and report surviving sessions on partial failure
• publish the extension through Wolfpack's Pi package manifest and document the workflow

closes #216

verification

• bun run typecheck
• full bun test: 1843 pass, 21 existing broker-dependent skips, 0 fail
• deterministic control-schema regeneration
• npm pack --dry-run --json: extension and docs present
• focused desktop grid Playwright: 2 passed
• real Pi RPC loader: /hunk registered, missing-Hunk notification emitted, zero model-turn events
• independent GPT-5.5 implementation review: ship, no actionable findings after remediation

manual gap

hunk is not installed on this host, so the final live Pi → Wolfpack grid → Hunk watch lifecycle was not exercised. The Pi loader, extension behavior, API, package, and existing grid notification boundaries are covered automatically.