story-kit: support agent assignment via story front matter (story 249)

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>

server/src/io/story_metadata.rs

@@ -7,6 +7,7 @@ pub struct StoryMetadata {
     pub name: Option<String>,
     pub coverage_baseline: Option<String>,
     pub merge_failure: Option<String>,
+    pub agent: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -29,6 +30,7 @@ struct FrontMatter {
     name: Option<String>,
     coverage_baseline: Option<String>,
     merge_failure: Option<String>,
+    agent: Option<String>,
 }
 
 pub fn parse_front_matter(contents: &str) -> Result<StoryMetadata, StoryMetaError> {
@@ -61,6 +63,7 @@ fn build_metadata(front: FrontMatter) -> StoryMetadata {
         name: front.name,
         coverage_baseline: front.coverage_baseline,
         merge_failure: front.merge_failure,
+        agent: front.agent,
     }
 }