fix: replace fast-forward with cherry-pick in mergemaster squash-merge

The mergemaster pipeline used git merge --ff-only to apply the squash
commit from a merge-queue branch onto master. This raced with the
filesystem watcher which auto-commits pipeline file moves to master,
causing the fast-forward to fail. The mergemaster agent would then
improvise by manually moving stories to done without the code merge.

- Replace --ff-only with cherry-pick so concurrent watcher commits
  don't block the merge
- Add report_merge_failure MCP tool for explicit failure handling
- Update mergemaster prompt to forbid manual file moves
- Fix cleanup_merge_workspace to handle stale directories

Squash merge of feature/story-205

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/http/mcp.rs

@@ -1,6 +1,7 @@
 use crate::agents::{close_bug_to_archive, move_story_to_archived, move_story_to_merge, move_story_to_qa, PipelineStage};
 use crate::config::ProjectConfig;
 use crate::log_buffer;
+use crate::slog;
 use crate::slog_warn;
 use crate::http::context::AppContext;
 use crate::http::settings::get_editor_command_from_store;
@@ -768,6 +769,24 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                         "required": ["story_id"]
                     }
                 },
+                {
+                    "name": "report_merge_failure",
+                    "description": "Report that a merge failed for a story. Leaves the story in work/4_merge/ and logs the failure reason. Use this when merge_agent_work returns success=false instead of manually moving the story file.",
+                    "inputSchema": {
+                        "type": "object",
+                        "properties": {
+                            "story_id": {
+                                "type": "string",
+                                "description": "Story identifier (e.g. '52_story_mergemaster_agent_role')"
+                            },
+                            "reason": {
+                                "type": "string",
+                                "description": "Human-readable explanation of why the merge failed"
+                            }
+                        },
+                        "required": ["story_id", "reason"]
+                    }
+                },
                 {
                     "name": "request_qa",
                     "description": "Trigger QA review of a completed story worktree: moves the item from work/2_current/ to work/3_qa/ and starts the qa agent to run quality gates, tests, and generate a manual testing plan.",
@@ -880,6 +899,7 @@ async fn handle_tools_call(
         // Mergemaster tools
         "merge_agent_work" => tool_merge_agent_work(&args, ctx).await,
         "move_story_to_merge" => tool_move_story_to_merge(&args, ctx).await,
+        "report_merge_failure" => tool_report_merge_failure(&args),
         // QA tools
         "request_qa" => tool_request_qa(&args, ctx).await,
         // Diagnostics
@@ -1568,11 +1588,11 @@ async fn tool_merge_agent_work(args: &Value, ctx: &AppContext) -> Result<String,
     } else if report.success && report.gates_passed {
         "Merge complete: all quality gates passed. Story moved to done and worktree cleaned up."
     } else if report.had_conflicts && !report.conflicts_resolved {
-        "Merge failed: conflicts detected that could not be auto-resolved. Merge was aborted — master is untouched. Report the conflict details so the human can resolve them."
+        "Merge failed: conflicts detected that could not be auto-resolved. Merge was aborted — master is untouched. Call report_merge_failure with the conflict details so the human can resolve them. Do NOT manually move the story file or call accept_story."
     } else if report.success && !report.gates_passed {
         "Merge committed but quality gates failed. Review gate_output and fix issues before re-running."
     } else {
-        "Merge failed. Review gate_output for details."
+        "Merge failed. Review gate_output for details. Call report_merge_failure to record the failure. Do NOT manually move the story file or call accept_story."
     };
 
     serde_json::to_string_pretty(&json!({
@@ -1625,6 +1645,23 @@ async fn tool_move_story_to_merge(args: &Value, ctx: &AppContext) -> Result<Stri
     .map_err(|e| format!("Serialization error: {e}"))
 }
 
+fn tool_report_merge_failure(args: &Value) -> Result<String, String> {
+    let story_id = args
+        .get("story_id")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: story_id")?;
+    let reason = args
+        .get("reason")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: reason")?;
+
+    slog!("[mergemaster] Merge failure reported for '{story_id}': {reason}");
+
+    Ok(format!(
+        "Merge failure for '{story_id}' recorded. Story remains in work/4_merge/. Reason: {reason}"
+    ))
+}
+
 // ── QA tool implementations ───────────────────────────────────────
 
 async fn tool_request_qa(args: &Value, ctx: &AppContext) -> Result<String, String> {
@@ -1895,10 +1932,11 @@ mod tests {
         assert!(names.contains(&"close_bug"));
         assert!(names.contains(&"merge_agent_work"));
         assert!(names.contains(&"move_story_to_merge"));
+        assert!(names.contains(&"report_merge_failure"));
         assert!(names.contains(&"request_qa"));
         assert!(names.contains(&"get_server_logs"));
         assert!(names.contains(&"prompt_permission"));
-        assert_eq!(tools.len(), 30);
+        assert_eq!(tools.len(), 31);
     }
 
     #[test]
@@ -2579,6 +2617,52 @@ mod tests {
         assert!(parsed.get("message").is_some());
     }
 
+    // ── report_merge_failure tool tests ─────────────────────────────
+
+    #[test]
+    fn report_merge_failure_in_tools_list() {
+        let resp = handle_tools_list(Some(json!(1)));
+        let tools = resp.result.unwrap()["tools"].as_array().unwrap().clone();
+        let tool = tools.iter().find(|t| t["name"] == "report_merge_failure");
+        assert!(
+            tool.is_some(),
+            "report_merge_failure missing from tools list"
+        );
+        let t = tool.unwrap();
+        assert!(t["description"].is_string());
+        let required = t["inputSchema"]["required"].as_array().unwrap();
+        let req_names: Vec<&str> = required.iter().map(|v| v.as_str().unwrap()).collect();
+        assert!(req_names.contains(&"story_id"));
+        assert!(req_names.contains(&"reason"));
+    }
+
+    #[test]
+    fn tool_report_merge_failure_missing_story_id() {
+        let result = tool_report_merge_failure(&json!({"reason": "conflicts"}));
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("story_id"));
+    }
+
+    #[test]
+    fn tool_report_merge_failure_missing_reason() {
+        let result = tool_report_merge_failure(&json!({"story_id": "42_story_foo"}));
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("reason"));
+    }
+
+    #[test]
+    fn tool_report_merge_failure_returns_confirmation() {
+        let result = tool_report_merge_failure(&json!({
+            "story_id": "42_story_foo",
+            "reason": "Unresolvable merge conflicts in src/main.rs"
+        }));
+        assert!(result.is_ok());
+        let msg = result.unwrap();
+        assert!(msg.contains("42_story_foo"));
+        assert!(msg.contains("work/4_merge/"));
+        assert!(msg.contains("Unresolvable merge conflicts"));
+    }
+
     // ── HTTP handler tests (TestClient) ───────────────────────────
 
     fn test_mcp_app(ctx: std::sync::Arc<AppContext>) -> impl poem::Endpoint {