refactor: split agents/pool/start.rs into mod.rs + validation.rs + spawn.rs

The 1630-line start.rs is split into a sub-module directory:

- validation.rs: validate_agent_stage + read_front_matter_agent helpers (69 lines)
- spawn.rs: run_agent_spawn — the background async work that was inlined as
  a tokio::spawn closure body inside start_agent (359 lines)
- mod.rs: AgentPool::start_agent orchestrator + tests (1062 lines)

Stage validation and front-matter agent reading are pre-lock pure helpers that
naturally extract.  The spawn closure body becomes a free async fn that takes
the previously-cloned values as parameters; rebound to the original _clone /
_owned names at the top of the body so the actual work code is a verbatim copy.

No behaviour change. All 23 start tests pass; full suite green.

server/src/agents/pool/start/validation.rs

@@ -0,0 +1,69 @@
+//! Pre-lock validation helpers for `AgentPool::start_agent`.
+
+use std::path::Path;
+
+use crate::config::ProjectConfig;
+
+use super::super::super::{PipelineStage, agent_config_stage, pipeline_stage};
+use super::super::worktree::find_active_story_stage;
+
+/// Validate that an explicit `agent_name` is allowed to attach to `story_id`'s
+/// current pipeline stage.
+///
+/// Prevents wrong-stage assignments like a mergemaster on a coding-stage story
+/// (bug 312).  Returns `Ok(())` if the agent has no specific stage (e.g.
+/// supervisor) or the story is not in an active stage; `Err` with a descriptive
+/// message on a stage mismatch.
+pub(super) fn validate_agent_stage(
+    config: &ProjectConfig,
+    project_root: &Path,
+    story_id: &str,
+    agent_name: Option<&str>,
+) -> Result<(), String> {
+    let Some(name) = agent_name else {
+        return Ok(());
+    };
+    let agent_stage = config
+        .find_agent(name)
+        .map(agent_config_stage)
+        .unwrap_or_else(|| pipeline_stage(name));
+    if agent_stage == PipelineStage::Other {
+        return Ok(());
+    }
+    let Some(story_stage_dir) = find_active_story_stage(project_root, story_id) else {
+        return Ok(());
+    };
+    let expected_stage = match story_stage_dir {
+        "2_current" => PipelineStage::Coder,
+        "3_qa" => PipelineStage::Qa,
+        "4_merge" => PipelineStage::Mergemaster,
+        _ => PipelineStage::Other,
+    };
+    if expected_stage != PipelineStage::Other && expected_stage != agent_stage {
+        return Err(format!(
+            "Agent '{name}' (stage: {agent_stage:?}) cannot be assigned to \
+             story '{story_id}' in {story_stage_dir}/ (requires stage: {expected_stage:?})"
+        ));
+    }
+    Ok(())
+}
+
+/// Read the preferred `agent:` field from the story's front matter.
+///
+/// When `agent_name` is `None` (caller is auto-selecting), this lets
+/// `start_agent` honour an explicit `agent: coder-opus` written by the
+/// `assign` command (bug 379).  Returns `None` when an explicit agent_name
+/// was already supplied or when the story has no front-matter preference.
+pub(super) fn read_front_matter_agent(
+    story_id: &str,
+    agent_name: Option<&str>,
+) -> Option<String> {
+    if agent_name.is_some() {
+        return None;
+    }
+    crate::db::read_content(story_id).and_then(|contents| {
+        crate::io::story_metadata::parse_front_matter(&contents)
+            .ok()?
+            .agent
+    })
+}