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/agents/pool.rs

@@ -1377,7 +1377,13 @@ impl AgentPool {
             for story_id in &items {
                 // Re-acquire the lock on each iteration to see state changes
                 // from previous start_agent calls in the same pass.
-                let (already_assigned, free_agent) = {
+                let preferred_agent =
+                    read_story_front_matter_agent(project_root, stage_dir, story_id);
+
+                // Outcome: (already_assigned, chosen_agent, preferred_busy)
+                // preferred_busy=true means the story has a specific agent requested but it is
+                // currently occupied — the story should wait rather than fall back.
+                let (already_assigned, free_agent, preferred_busy) = {
                     let agents = match self.agents.lock() {
                         Ok(a) => a,
                         Err(e) => {
@@ -1386,13 +1392,20 @@ impl AgentPool {
                         }
                     };
                     let assigned = is_story_assigned_for_stage(&config, &agents, story_id, stage);
-                    let free = if assigned {
+                    if assigned {
-                        None
+                        (true, None, false)
+                    } else if let Some(ref pref) = preferred_agent {
+                        // Story has a front-matter agent preference.
+                        if is_agent_free(&agents, pref) {
+                            (false, Some(pref.clone()), false)
+                        } else {
+                            (false, None, true)
+                        }
                     } else {
-                        find_free_agent_for_stage(&config, &agents, stage)
+                        let free = find_free_agent_for_stage(&config, &agents, stage)
-                            .map(|s| s.to_string())
+                            .map(|s| s.to_string());
-                    };
+                        (false, free, false)
-                    (assigned, free)
+                    }
                 };
 
                 if already_assigned {
@@ -1400,6 +1413,16 @@ impl AgentPool {
                     continue;
                 }
 
+                if preferred_busy {
+                    // The story requests a specific agent that is currently busy.
+                    // Do not fall back to a different agent; let this story wait.
+                    slog!(
+                        "[auto-assign] Preferred agent '{}' busy for '{story_id}'; story will wait.",
+                        preferred_agent.as_deref().unwrap_or("?")
+                    );
+                    continue;
+                }
+
                 match free_agent {
                     Some(agent_name) => {
                         slog!(
@@ -1815,6 +1838,29 @@ fn find_active_story_stage(project_root: &Path, story_id: &str) -> Option<&'stat
 
 /// Scan a work pipeline stage directory and return story IDs, sorted alphabetically.
 /// Returns an empty `Vec` if the directory does not exist.
+/// Read the optional `agent:` field from the front matter of a story file.
+///
+/// Returns `Some(agent_name)` if the front matter specifies an agent, or `None`
+/// if the field is absent or the file cannot be read / parsed.
+fn read_story_front_matter_agent(project_root: &Path, stage_dir: &str, story_id: &str) -> Option<String> {
+    use crate::io::story_metadata::parse_front_matter;
+    let path = project_root
+        .join(".story_kit")
+        .join("work")
+        .join(stage_dir)
+        .join(format!("{story_id}.md"));
+    let contents = std::fs::read_to_string(path).ok()?;
+    parse_front_matter(&contents).ok()?.agent
+}
+
+/// Return `true` if `agent_name` has no active (pending/running) entry in the pool.
+fn is_agent_free(agents: &HashMap<String, StoryAgent>, agent_name: &str) -> bool {
+    !agents.values().any(|a| {
+        a.agent_name == agent_name
+            && matches!(a.status, AgentStatus::Running | AgentStatus::Pending)
+    })
+}
+
 fn scan_stage_items(project_root: &Path, stage_dir: &str) -> Vec<String> {
     let dir = project_root
         .join(".story_kit")

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,
     }
 }