chore: add MERGE-DEBUG traces for project_root lifecycle

Temporary diagnostic logging to track why project_root becomes None
during merge pipeline operations. Tagged with MERGE-DEBUG for easy
grep-and-remove once the root cause is confirmed fixed.

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

server/src/http/mcp.rs

@@ -1580,7 +1580,17 @@ async fn tool_merge_agent_work(args: &Value, ctx: &AppContext) -> Result<String,
         .ok_or("Missing required argument: story_id")?;
     let agent_name = args.get("agent_name").and_then(|v| v.as_str());
 
+    // TRACE:MERGE-DEBUG — remove once root cause is found
+    crate::slog!(
+        "[MERGE-DEBUG] tool_merge_agent_work called for story_id={:?}, agent_name={:?}",
+        story_id,
+        agent_name
+    );
     let project_root = ctx.agents.get_project_root(&ctx.state)?;
+    crate::slog!(
+        "[MERGE-DEBUG] tool_merge_agent_work: project_root resolved to {:?}",
+        project_root
+    );
     let report = ctx.agents.merge_agent_work(&project_root, story_id).await?;
 
     let status_msg = if report.success && report.gates_passed && report.conflicts_resolved {

server/src/http/project.rs

@@ -49,6 +49,11 @@ impl ProjectApi {
     /// Close the current project and clear the stored selection.
     #[oai(path = "/project", method = "delete")]
     async fn close_project(&self) -> OpenApiResult<Json<bool>> {
+        // TRACE:MERGE-DEBUG — remove once root cause is found
+        crate::slog_error!(
+            "[MERGE-DEBUG] DELETE /project called! \
+             Backtrace: this is the only code path that clears project_root."
+        );
         fs::close_project(&self.ctx.state, self.ctx.store.as_ref()).map_err(bad_request)?;
         Ok(Json(true))
     }

server/src/io/fs.rs

@@ -532,6 +532,8 @@ pub async fn open_project(
     let _ = worktree_write_mcp_json(&p, port);
 
     {
+        // TRACE:MERGE-DEBUG — remove once root cause is found
+        crate::slog!("[MERGE-DEBUG] open_project: setting project_root to {:?}", p);
         let mut root = state.project_root.lock().map_err(|e| e.to_string())?;
         *root = Some(p);
     }
@@ -551,6 +553,8 @@ pub async fn open_project(
 
 pub fn close_project(state: &SessionState, store: &dyn StoreOps) -> Result<(), String> {
     {
+        // TRACE:MERGE-DEBUG — remove once root cause is found
+        crate::slog!("[MERGE-DEBUG] close_project: setting project_root to None");
         let mut root = state.project_root.lock().map_err(|e| e.to_string())?;
         *root = None;
     }
@@ -579,6 +583,12 @@ pub fn get_current_project(
     {
         let p = PathBuf::from(path_str);
         if p.exists() && p.is_dir() {
+            // TRACE:MERGE-DEBUG — remove once root cause is found
+            crate::slog!(
+                "[MERGE-DEBUG] get_current_project: project_root was None, \
+                 restoring from store to {:?}",
+                p
+            );
             let mut root = state.project_root.lock().map_err(|e| e.to_string())?;
             *root = Some(p);
             return Ok(Some(path_str.to_string()));

server/src/main.rs

@@ -94,6 +94,8 @@ async fn main() -> Result<(), std::io::Error> {
                 .unwrap_or_else(|e| panic!("Invalid project.toml: {e}"));
         } else {
             // No .story_kit/ found — fall back to cwd so existing behaviour is preserved.
+            // TRACE:MERGE-DEBUG — remove once root cause is found
+            slog!("[MERGE-DEBUG] main: no .story_kit/ found, falling back to cwd {:?}", cwd);
             *app_state.project_root.lock().unwrap() = Some(cwd.clone());
         }
     }

server/src/state.rs

@@ -22,9 +22,15 @@ impl Default for SessionState {
 impl SessionState {
     pub fn get_project_root(&self) -> Result<PathBuf, String> {
         let root_guard = self.project_root.lock().map_err(|e| e.to_string())?;
-        let root = root_guard
-            .as_ref()
-            .ok_or_else(|| "No project is currently open.".to_string())?;
+        let root = root_guard.as_ref().ok_or_else(|| {
+            // TRACE:MERGE-DEBUG — remove once root cause is found
+            crate::slog_error!(
+                "[MERGE-DEBUG] get_project_root() called but project_root is None! \
+                 Backtrace hint: check caller in MCP tool handler."
+            );
+            "No project is currently open.".to_string()
+        })?;
         Ok(root.clone())
     }
+
 }