[plan] Update compiler to emit unlock-issue step in conclusion job

[github-actions]

Objective

Modify the workflow compiler to emit a step that unlocks the issue in the conclusion job, ensuring it runs even on failure.

Context

Depends on #6202, #aw_ghi789jkl012, #aw_jkl012mno345 being completed. The conclusion job must unlock the issue regardless of agent success or failure.

Approach

1. In pkg/workflow/compiler.go or the conclusion job builder, check if on.issue.lock-for-agent is true
2. If enabled, emit a step in the conclusion job that runs unlock-issue.cjs script
3. Ensure the step runs with if: always() condition so it executes even on failure
4. Use the bundled script reference (similar to how other embedded scripts are used)
5. Test by compiling a workflow with lock-for-agent: true and verify the generated YAML

Files to Modify

• pkg/workflow/compiler.go or related conclusion job builder files
• Test compilation with sample workflow

Acceptance Criteria

• Compiler checks on.issue.lock-for-agent field from frontmatter
• When true, conclusion job includes unlock-issue step
• Step includes if: always() to run even on failure
• Step executes in the conclusion job
• Compiled workflow YAML includes the correct step structure
• Manual test confirms issue unlocks after workflow completes or fails
  Related to Locking of issues #6201

AI generated by Plan Command for #6201