Remove _WorkflowCommon module (cleanup)

[devloop-bot]

Parent

Parent: #196

What to build

Delete the _WorkflowCommon module and remove all imports. With DevLoopWorkflow and PRCommentWorkflow both migrated to PhaseOps in #197/#198/#199, this file has no callers left.

This is the cleanup slice — it removes the dead code and restores the codebase to a single I/O seam.

Acceptance criteria

• _workflow_common.py is deleted
• No imports of _workflow_common remain in the codebase
• All existing tests still pass
• CI pipeline passes end-to-end
• sentrux quality score unchanged or improved

Blocked by

• Update DevLoopWorkflow to use PhaseOps I/O methods #198 (Update DevLoopWorkflow to use PhaseOps I/O methods)
• Update PRCommentWorkflow to use PhaseOps I/O methods #199 (Update PRCommentWorkflow to use PhaseOps I/O methods)