feat: detect stale config and prompt merge-or-overwrite on save

Records the file mtime at load time; before write_config() fires, re-checks
it. If it changed (e.g. `claude mcp add`, a second BCC window, or Claude
itself writing ~/.claude.json), StaleDialog prompts with the changed top-level
key names and a masked server-section diff. "Merge & save" applies the user's
in-memory server edits on top of the current on-disk file (preserving external
non-server changes); "Overwrite anyway" proceeds as before.

- bcc_core: config_mtime(), external_change_summary(), _server_sections()
  helper extracted from backup_diff for reuse
- bcc.py: StaleDialog, MainWindow._loaded_mtime tracked through load/save
- tests: 7 new tests (config_mtime, external_change_summary variants, AC merge test)

Closes #4

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

bcc_core.py

@@ -290,6 +290,14 @@ def _redact_servers_block(block: dict | None) -> dict:
     return {name: _redact_server_data(data) for name, data in block.items()}
 
 
+def _server_sections(cfg: dict) -> dict:
+    """Return the masked server sections of a config dict, safe for diff display."""
+    out: dict = {"mcpServers": _redact_servers_block(cfg.get("mcpServers"))}
+    if DISABLED_KEY in cfg:
+        out[DISABLED_KEY] = _redact_servers_block(cfg.get(DISABLED_KEY))
+    return out
+
+
 def backup_diff(
     config_path: Path | str,
     backup_path: Path | str,
@@ -318,12 +326,6 @@ def backup_diff(
     after_cfg = dict(current_cfg)
     apply_servers(after_cfg, backup_servers)
 
-    def _server_sections(cfg: dict) -> dict:
-        out: dict = {"mcpServers": _redact_servers_block(cfg.get("mcpServers"))}
-        if DISABLED_KEY in cfg:
-            out[DISABLED_KEY] = _redact_servers_block(cfg.get(DISABLED_KEY))
-        return out
-
     before_lines = (
         json.dumps(_server_sections(current_cfg), indent=2, ensure_ascii=False) + "\n"
     ).splitlines(keepends=True)
@@ -370,6 +372,46 @@ def restore_backup(
     return write_config(p, cfg)
 
 
+def config_mtime(path: Path | str) -> float | None:
+    """Return the file's mtime, or None if the file does not exist."""
+    try:
+        return Path(path).stat().st_mtime
+    except OSError:
+        return None
+
+
+def external_change_summary(original_cfg: dict, path: Path | str) -> tuple[list[str], str]:
+    """
+    Compare original_cfg (what BCC loaded) with the current on-disk state.
+
+    Returns:
+      changed_keys — sorted list of top-level keys whose values differ.
+      server_diff  — masked unified diff of server sections (empty if unchanged
+                     or the file cannot be read).
+    """
+    try:
+        disk_cfg = load_config(Path(path))
+    except Exception:
+        return [], ""
+
+    all_keys = set(original_cfg) | set(disk_cfg)
+    changed_keys = sorted(k for k in all_keys if original_cfg.get(k) != disk_cfg.get(k))
+
+    server_diff = ""
+    if any(k in {"mcpServers", DISABLED_KEY} for k in changed_keys):
+        before_lines = (
+            json.dumps(_server_sections(original_cfg), indent=2, ensure_ascii=False) + "\n"
+        ).splitlines(keepends=True)
+        after_lines = (
+            json.dumps(_server_sections(disk_cfg), indent=2, ensure_ascii=False) + "\n"
+        ).splitlines(keepends=True)
+        server_diff = "".join(
+            difflib.unified_diff(before_lines, after_lines, fromfile="loaded", tofile="on disk now")
+        )
+
+    return changed_keys, server_diff
+
+
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