Adds an optional `agent:` field to story file front matter so that a
specific agent can be requested for a story. The auto-assign loop now:
1. Reads the front-matter `agent` field for each story before picking
a free agent.
2. If a preferred agent is named, uses it when free; skips the story
(without falling back) when that agent is busy.
3. Falls back to the existing `find_free_agent_for_stage` behaviour
when no preference is specified.
Ported from feature branch that predated the agents.rs module refactoring.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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