Refactor agents.rs (7631 lines) into agents/ module directory

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>

server/src/agents/mod.rs

@@ -0,0 +1,181 @@
+pub mod gates;
+pub mod lifecycle;
+pub mod merge;
+mod pool;
+mod pty;
+
+use crate::config::AgentConfig;
+use serde::Serialize;
+
+pub use lifecycle::{
+    close_bug_to_archive, feature_branch_has_unmerged_changes, move_story_to_archived,
+    move_story_to_merge, move_story_to_qa,
+};
+pub use pool::AgentPool;
+
+/// Events emitted during server startup reconciliation to broadcast real-time
+/// progress to connected WebSocket clients.
+#[derive(Debug, Clone, Serialize)]
+pub struct ReconciliationEvent {
+    /// The story being reconciled, or empty string for the overall "done" event.
+    pub story_id: String,
+    /// Coarse status: "checking", "gates_running", "advanced", "skipped", "failed", "done"
+    pub status: String,
+    /// Human-readable details.
+    pub message: String,
+}
+
+/// Events streamed from a running agent to SSE clients.
+#[derive(Debug, Clone, Serialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum AgentEvent {
+    /// Agent status changed.
+    Status {
+        story_id: String,
+        agent_name: String,
+        status: String,
+    },
+    /// Raw text output from the agent process.
+    Output {
+        story_id: String,
+        agent_name: String,
+        text: String,
+    },
+    /// Agent produced a JSON event from `--output-format stream-json`.
+    AgentJson {
+        story_id: String,
+        agent_name: String,
+        data: serde_json::Value,
+    },
+    /// Agent finished.
+    Done {
+        story_id: String,
+        agent_name: String,
+        session_id: Option<String>,
+    },
+    /// Agent errored.
+    Error {
+        story_id: String,
+        agent_name: String,
+        message: String,
+    },
+    /// Thinking tokens from an extended-thinking block.
+    Thinking {
+        story_id: String,
+        agent_name: String,
+        text: String,
+    },
+}
+
+#[derive(Debug, Clone, Serialize, PartialEq)]
+#[serde(rename_all = "snake_case")]
+pub enum AgentStatus {
+    Pending,
+    Running,
+    Completed,
+    Failed,
+}
+
+impl std::fmt::Display for AgentStatus {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Pending => write!(f, "pending"),
+            Self::Running => write!(f, "running"),
+            Self::Completed => write!(f, "completed"),
+            Self::Failed => write!(f, "failed"),
+        }
+    }
+}
+
+/// Pipeline stages for automatic story advancement.
+#[derive(Debug, Clone, PartialEq)]
+pub enum PipelineStage {
+    /// Coding agents (coder-1, coder-2, etc.)
+    Coder,
+    /// QA review agent
+    Qa,
+    /// Mergemaster agent
+    Mergemaster,
+    /// Supervisors and unknown agents — no automatic advancement.
+    Other,
+}
+
+/// Determine the pipeline stage from an agent name.
+pub fn pipeline_stage(agent_name: &str) -> PipelineStage {
+    match agent_name {
+        "qa" => PipelineStage::Qa,
+        "mergemaster" => PipelineStage::Mergemaster,
+        name if name.starts_with("coder") => PipelineStage::Coder,
+        _ => PipelineStage::Other,
+    }
+}
+
+/// Determine the pipeline stage for a configured agent.
+///
+/// Prefers the explicit `stage` config field (added in Bug 150) over the
+/// legacy name-based heuristic so that agents with non-standard names
+/// (e.g. `qa-2`, `coder-opus`) are assigned to the correct stage.
+pub(crate) fn agent_config_stage(cfg: &AgentConfig) -> PipelineStage {
+    match cfg.stage.as_deref() {
+        Some("coder") => PipelineStage::Coder,
+        Some("qa") => PipelineStage::Qa,
+        Some("mergemaster") => PipelineStage::Mergemaster,
+        Some(_) => PipelineStage::Other,
+        None => pipeline_stage(&cfg.name),
+    }
+}
+
+/// Completion report produced when acceptance gates are run.
+///
+/// Created automatically by the server when an agent process exits normally,
+/// or via the internal `report_completion` method.
+#[derive(Debug, Serialize, Clone)]
+pub struct CompletionReport {
+    pub summary: String,
+    pub gates_passed: bool,
+    pub gate_output: String,
+}
+
+#[derive(Debug, Serialize, Clone)]
+pub struct AgentInfo {
+    pub story_id: String,
+    pub agent_name: String,
+    pub status: AgentStatus,
+    pub session_id: Option<String>,
+    pub worktree_path: Option<String>,
+    pub base_branch: Option<String>,
+    pub completion: Option<CompletionReport>,
+    /// UUID identifying the persistent log file for this session.
+    pub log_session_id: Option<String>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // ── pipeline_stage tests ──────────────────────────────────────────────────
+
+    #[test]
+    fn pipeline_stage_detects_coders() {
+        assert_eq!(pipeline_stage("coder-1"), PipelineStage::Coder);
+        assert_eq!(pipeline_stage("coder-2"), PipelineStage::Coder);
+        assert_eq!(pipeline_stage("coder-3"), PipelineStage::Coder);
+    }
+
+    #[test]
+    fn pipeline_stage_detects_qa() {
+        assert_eq!(pipeline_stage("qa"), PipelineStage::Qa);
+    }
+
+    #[test]
+    fn pipeline_stage_detects_mergemaster() {
+        assert_eq!(pipeline_stage("mergemaster"), PipelineStage::Mergemaster);
+    }
+
+    #[test]
+    fn pipeline_stage_supervisor_is_other() {
+        assert_eq!(pipeline_stage("supervisor"), PipelineStage::Other);
+        assert_eq!(pipeline_stage("default"), PipelineStage::Other);
+        assert_eq!(pipeline_stage("unknown"), PipelineStage::Other);
+    }
+}