fix: self-ignore the .nightcrow session directory

The session writer creates `<repo>/.nightcrow/session.json` inside the
user's working tree. Without explicit setup the file appears as an
untracked path in both `git status` and nightcrow's own status view,
which is noise the user has to suppress per-repo by editing their
`.gitignore`. Have `save_session` drop a `.nightcrow/.gitignore`
containing `*` on first write so the directory ignores itself; an
existing file is left alone so a user-customised ignore pattern is
never clobbered.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: baudouin-iv

src/session.rs

@@ -44,10 +44,19 @@ pub fn load_session(repo_path: &str) -> Option<SessionState> {
 
 pub fn save_session(repo_path: &str, state: &SessionState) {
     let path = session_path(repo_path);
-    if let Some(dir) = path.parent()
-        && let Err(e) = std::fs::create_dir_all(dir)
-    {
-        tracing::warn!("failed to create session directory: {e}");
+    if let Some(dir) = path.parent() {
+        if let Err(e) = std::fs::create_dir_all(dir) {
+            tracing::warn!("failed to create session directory: {e}");
+        }
+        // Drop a self-ignoring `.gitignore` inside `.nightcrow/` so the
+        // session file never pollutes the user's `git status`. Only write
+        // when missing — a user-edited file should not be clobbered.
+        let gi = dir.join(".gitignore");
+        if !gi.exists()
+            && let Err(e) = std::fs::write(&gi, "*\n")
+        {
+            tracing::warn!("failed to write nightcrow gitignore: {e}");
+        }
     }
     let text = match serde_json::to_string(state) {
         Ok(t) => t,