Deduplicate concurrent GitHub webhook retries

[wellkilo]

Summary

• make maintenance-task creation idempotent per GitHub delivery ID
• share one in-flight creation across concurrent webhook retries
• dispatch only the newly created task
• preserve independent processing for different delivery IDs
• document the webhook replay scenario and repair contract

Root cause

The processor performed find and save as separate async operations. Two concurrent requests could both observe an empty store, create different task IDs, and dispatch twice.

Repair

DeliveryTaskStore.getOrCreate now owns the entire create-or-reuse decision. It caches the in-flight Promise for each delivery ID, so concurrent retries await the same task. Completed tasks are retained for sequential retries; the in-flight entry is always cleared after completion.

Verification

npm ci
npm run typecheck
npm test
npm run build

Validated locally:

• 7 tests passed
• concurrent same-delivery requests return the same task ID
• exactly one result is newly created
• dispatch runs once
• sequential retry reuses the task
• different delivery IDs remain independent

Relates to #3

Created as the RepoPilot Issue-to-PR demo under the pull_request_only policy. This PR must remain open for human review and must not be auto-merged.