refactor: split agents/merge.rs into mod.rs + squash.rs + conflicts.rs

The 1772-line merge.rs is split into:

- conflicts.rs: try_resolve_conflicts + resolve_simple_conflicts + tests (351 lines)
- squash.rs: run_squash_merge orchestrator + cleanup + run_merge_quality_gates + tests (1306 lines)
- mod.rs: doc, types (MergeJobStatus, MergeJob, MergeReport, SquashMergeResult), re-exports (52 lines)

Tests stay co-located.

No behaviour change. All 20 merge tests pass; full suite green
(2635 tests with --test-threads=1).

server/src/agents/merge/mod.rs

@@ -0,0 +1,52 @@
+//! Merge operations — rebases agent work onto master and runs post-merge validation.
+
+use serde::Serialize;
+
+mod conflicts;
+mod squash;
+
+pub(crate) use squash::{cleanup_merge_workspace, run_squash_merge};
+
+/// Status of an async merge job.
+#[derive(Debug, Clone, Serialize)]
+pub enum MergeJobStatus {
+    Running,
+    Completed(MergeReport),
+    Failed(String),
+}
+
+/// Tracks a background merge job started by `merge_agent_work`.
+#[derive(Debug, Clone, Serialize)]
+pub struct MergeJob {
+    pub story_id: String,
+    pub status: MergeJobStatus,
+}
+
+/// Result of a mergemaster merge operation.
+#[derive(Debug, Serialize, Clone)]
+pub struct MergeReport {
+    pub story_id: String,
+    pub success: bool,
+    pub had_conflicts: bool,
+    /// `true` when conflicts were detected but automatically resolved.
+    pub conflicts_resolved: bool,
+    pub conflict_details: Option<String>,
+    pub gates_passed: bool,
+    pub gate_output: String,
+    pub worktree_cleaned_up: bool,
+    pub story_archived: bool,
+}
+
+/// Result of a squash-merge operation.
+pub(crate) struct SquashMergeResult {
+    pub(crate) success: bool,
+    pub(crate) had_conflicts: bool,
+    /// `true` when conflicts were detected but automatically resolved.
+    pub(crate) conflicts_resolved: bool,
+    pub(crate) conflict_details: Option<String>,
+    pub(crate) output: String,
+    /// Whether quality gates ran and passed.  `false` when `success` is `false`
+    /// due to a gate failure; callers can use this to distinguish gate failures
+    /// from merge/commit/FF failures in the `MergeReport`.
+    pub(crate) gates_passed: bool,
+}