fix: stop stagnant todo continuation loops

[code-yeongyu]

Fixes #2322

Summary

• add stagnation tracking to todo continuation sessions and stop re-injecting after three no-progress continuation cycles
• reset stagnation state when todo progress resumes or all todos are cleared, using a dedicated helper to keep idle-event gating focused
• add regression coverage for stagnation tracking and update failure-backoff tests so they continue exercising the failure path without tripping the new stagnation guard

Testing

• bun test src/hooks/todo-continuation-enforcer/
• bun run typecheck
• bun run build

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Summary by cubic

Prevents idle todo continuation from looping when todos don’t change. Adds stagnation tracking and stops re-injection after three no-progress cycles. Addresses Linear #2322.

• Bug Fixes
  • Track incomplete count and stagnation per session; reset on progress or when todos clear.
  • Stop reinjecting after 3 stagnant cycles using a new guard (MAX_STAGNATION_COUNT).
  • Add regression tests and adjust failure-backoff tests to avoid the new guard.

^{Written for commit fe12fc6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Implements stagnation tracking to prevent infinite loops in todo continuation. Includes comprehensive tests and logic to reset tracking when progress is made or todos are cleared.}