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>

tests/test_core.py

@@ -500,3 +500,86 @@ def test_restore_backup_restores_disabled_servers(tmp_path):
     assert "parked" in restored.get(c.DISABLED_KEY, {})
     assert "active" in restored.get("mcpServers", {})
     assert "newcomer" not in restored.get("mcpServers", {})
+
+
+# --------------------------------------------------------------------------- #
+# 9 — Stale-file protection
+# --------------------------------------------------------------------------- #
+
+
+def test_config_mtime_returns_float_for_existing_file(tmp_path):
+    f = tmp_path / "cfg.json"
+    f.write_text("{}")
+    assert isinstance(c.config_mtime(f), float)
+
+
+def test_config_mtime_returns_none_for_missing_file(tmp_path):
+    assert c.config_mtime(tmp_path / "nonexistent.json") is None
+
+
+def test_external_change_summary_detects_non_server_key_change(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    original = {"numStartups": 1, "mcpServers": {"s": {"command": "node"}}}
+    cfgpath.write_text(json.dumps({"numStartups": 2, "mcpServers": {"s": {"command": "node"}}}))
+    changed_keys, server_diff = c.external_change_summary(original, cfgpath)
+    assert "numStartups" in changed_keys
+    assert server_diff == ""  # server sections unchanged
+
+
+def test_external_change_summary_detects_server_section_change(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    original = {"mcpServers": {"s1": {"command": "node"}}}
+    cfgpath.write_text(json.dumps({"mcpServers": {"s2": {"command": "python"}}}))
+    changed_keys, server_diff = c.external_change_summary(original, cfgpath)
+    assert "mcpServers" in changed_keys
+    assert "s1" in server_diff or "s2" in server_diff
+
+
+def test_external_change_summary_server_diff_masks_secrets(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    original = {"mcpServers": {"a": {"command": "node", "env": {"SECRET_KEY": "s3cr3t"}}}}
+    # command changes (visible in diff) AND secret value changes (both sides masked →
+    # MASK appears in context lines, raw secret never appears)
+    cfgpath.write_text(
+        json.dumps(
+            {"mcpServers": {"a": {"command": "python", "env": {"SECRET_KEY": "n3w_s3cr3t"}}}}
+        )
+    )
+    changed_keys, server_diff = c.external_change_summary(original, cfgpath)
+    assert "mcpServers" in changed_keys
+    assert "s3cr3t" not in server_diff
+    assert "n3w_s3cr3t" not in server_diff
+    assert c.MASK in server_diff  # appears in context for the masked env value
+
+
+def test_external_change_summary_empty_when_unchanged(tmp_path):
+    cfgpath = tmp_path / "cfg.json"
+    cfg = {"mcpServers": {"s": {"command": "node"}}}
+    cfgpath.write_text(json.dumps(cfg))
+    changed_keys, server_diff = c.external_change_summary(cfg, cfgpath)
+    assert changed_keys == []
+    assert server_diff == ""
+
+
+def test_merge_and_reapply_preserves_both_edits(tmp_path):
+    """AC test: external non-server change + in-memory server edit both survive merge."""
+    cfgpath = tmp_path / "cfg.json"
+    original = {"numStartups": 1, "mcpServers": {"s1": {"command": "node"}}}
+    cfgpath.write_text(json.dumps(original, indent=2))
+
+    user_servers = [c.ServerEntry("s2", {"command": "python"}, True)]
+
+    # External process bumps numStartups without touching servers
+    disk_changed = dict(original)
+    disk_changed["numStartups"] = 42
+    cfgpath.write_text(json.dumps(disk_changed, indent=2))
+
+    # Merge: load fresh from disk, apply user's server edits, write back
+    fresh = c.load_config(cfgpath)
+    c.apply_servers(fresh, user_servers)
+    c.write_config(cfgpath, fresh)
+
+    result = json.loads(cfgpath.read_text())
+    assert result["numStartups"] == 42  # external change preserved
+    assert "s2" in result["mcpServers"]  # user's server edit preserved
+    assert "s1" not in result["mcpServers"]  # old server replaced