[plan] Add staged/dry-run testing pattern to developer.instructions

[github-actions]

Objective

Standardize workflow testing guidance by adding a "staged mode" or "dry-run" pattern to the agentic-workflows agent instructions.

Context

From discussion #12461: Only 3/8 workflows mentioned testing approaches (staged mode, manual trigger, comment-only mode). This leads to inconsistent testing guidance and potential production incidents from untested workflows.

Approach

1. Add a new section to .github/instructions/developer.instructions.md covering workflow testing best practices

2. Document the following testing patterns:

   • Staged mode: Comment-only execution without side effects
   • Manual triggers: Using workflow_dispatch for initial testing
   • Comment-only mode: Test safe-outputs without creating actual issues/PRs
   • Dry-run flags: Environment variables or inputs to simulate execution

3. Include examples from existing workflows:

   • QA-1 test coverage (staged mode)
   • PM-2 release notes (comment-only mode)
   • BE-1 database migration (manual trigger first)

Files to Modify

• Update: .github/instructions/developer.instructions.md
  • Add "Workflow Testing Best Practices" section
  • Include code examples for each testing pattern
  • Add recommendation: "Always suggest a testing approach for new workflows"

Acceptance Criteria

• New section documents all 4 testing patterns
• Each pattern includes a code example
• Section is positioned prominently in the instructions (before "Examples" section)
• Instructions tell agent to proactively suggest testing approaches
• Examples reference real workflows from the codebase

AI generated by Plan Command for discussion #12461