fix(pi-subagents): remove subagent timeouts

[tasercake]

Summary

• remove the timeout option from spawn_subagent
• remove timeout timers, persisted timeout state, status rendering, retries, and timeout notifications
• remove timeout documentation and timeout-specific tests while preserving completion, failure, cohort, widget, lifeline, and recursive-spawn behavior

Validation

• npm test — 53 passed, 0 failed, 1 opt-in real-provider integration test skipped
• strict source-only TypeScript check — passed
• git diff --check — passed
• targeted repository search found no remaining subagent-timeout semantics

Full-project tsc still reports three pre-existing MockPiResponse.stopReason test typing errors; this extension has no package typecheck script.