fix(background-agent): prevent formatForCompaction crash on undefined description

[FFFergie]

Summary

• Prevent TaskHistory.formatForCompaction() from crashing when a task entry has an undefined description.
• Keep compaction output formatting stable by defaulting missing descriptions to an empty string.
• Add a regression test that reproduces the undefined-description scenario.

Changes

• Updated src/features/background-agent/task-history.ts:
  • Guarded description sanitization with (e.description ?? "") before .replace().
• Updated src/features/background-agent/task-history.test.ts:
  • Added handles entries with undefined description without throwing test case.

Testing

bun test src/features/background-agent/task-history.test.ts
bun run typecheck
bun run build

Related Issues

Closes #1991

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Summary by cubic

Fix crash when formatting task history entries with undefined descriptions by treating them as empty strings. Adds a regression test; addresses #1991.

^{Written for commit 59991a4. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

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

_{Auto-approved: cubic found no issues; change adds guard for undefined description preventing crash with regression tests included.}