Add max turn time watchdog

[danyel117]

Summary

• add an opt-in max_turn_time watchdog that retries one goal prompt when a model step remains busy past the configured duration
• preserve Task and restricted-agent safety while keeping watchdog retries outside auto-turn, no-progress, and prompt-failure budgets
• cancel and clean up watchdogs on idle, built-in retry, session deletion, and plugin disposal
• correct the README attribution from Willy Topete to William Ricchiuti

Validation

• bun run lint
• bun run typecheck
• bun test (69 passed)
• bun run build
• bun run pack:dry-run
• independent read-only review with Claude Opus: approved with no remaining findings

Closes #16