Prevent duplicate PR tasks and add update-pr-description task type

[newhook]

Summary

This PR fixes an issue where multiple PR tasks were being created during review-fix loops, leading to duplicate work. Now:

• Before creating a PR task, the system checks if one already exists for the work
• If a pending/processing PR task exists, no new task is created
• If a completed PR task exists (PR was already created), an update-pr-description task is created instead to update the existing PR's description with latest changes
• Failed PR tasks are excluded from the check, allowing retry

Changes

Core Logic (cmd/orchestrate.go)

• Enhanced createPRTask() to check for existing PR tasks before creating new ones
• Added createUpdatePRDescriptionTask() for updating existing PR descriptions after subsequent reviews

Database Layer

• sql/queries/tasks.sql: Added GetPRTaskForWork query to find PR tasks by work ID
• internal/db/sqlc/: Regenerated SQLC code
• internal/db/task.go: Added GetPRTaskForWork() wrapper returning *Task or nil

Tests

• internal/db/task_test.go: Unit tests for the new database function
  • Tests basic retrieval across pending/processing/completed states
  • Verifies failed PR tasks are not returned
  • Ensures work isolation (PR task from work-1 not returned for work-2)
• cmd/orchestrate_test.go: Integration tests for PR task creation flow
  • No existing PR task → creates PR task
  • Pending PR task exists → skips creation
  • Processing PR task exists → skips creation
  • Completed PR task exists → creates update-pr-description task
  • Failed PR task → allows new PR task
  • Review-fix loop creates only one PR task
  • Multiple review cycles create multiple update-pr-description tasks

Issues Resolved

• ✅ ac-8dmv: Preventing duplicate PR tasks
• ✅ ac-8dmv.1: Add check for existing PR task before creating new one
• ✅ ac-8dmv.2: Add update_pr task type for updating PR descriptions
• ✅ ac-8dmv.3: Add unit and integration tests for PR task creation flow

Test Plan

• All existing tests pass
• New unit tests for GetPRTaskForWork database function
• New integration tests covering all PR task creation scenarios
• Manual testing: Run co work review multiple times on a work with an existing PR

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