Split the monolithic agents.rs into 6 focused modules:
- mod.rs: shared types (AgentEvent, AgentStatus, etc.) and re-exports
- pool.rs: AgentPool struct, all methods, and helper free functions
- pty.rs: PTY streaming (run_agent_pty_blocking, emit_event)
- lifecycle.rs: story movement functions (move_story_to_qa, etc.)
- gates.rs: acceptance gates (clippy, tests, coverage)
- merge.rs: squash-merge, conflict resolution, quality gates
All 121 original tests are preserved and distributed across modules.
Also adds clear_front_matter_field to story_metadata.rs to strip
stale merge_failure from front matter when stories move to done.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
