✨ feat(kebab-mcp): doctor tool (fb-30)

Second tool — `doctor` (no input args, returns doctor.v1 JSON via
kebab_app::doctor_with_config_path). Mirrors schema tool's manual-dispatch
pattern: Tool::new entry in list_tools, match arm in call_tool, per-tool
module in tools/doctor.rs.

doctor_with_config_path takes Option<&Path> (not &Config), so KebabAppState
is extended with config_path: Option<PathBuf>. All existing callers
(initialize.rs, tools_call_schema.rs, serve_stdio_async) pass None for now;
Plan Task 10 (Cmd::Mcp wiring) will thread the actual --config path through.
doctor_with_config falls back to XDG default when config_path is None —
same behavior as bare `kebab doctor`.

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

crates/kebab-mcp/tests/tools_call_schema.rs

@@ -39,7 +39,7 @@ async fn schema_tool_returns_schema_v1_json() {
     };
     let _ = kebab_app::ingest_with_config(config.clone(), scope, false).unwrap();
 
-    let state = KebabAppState::new(config);
+    let state = KebabAppState::new(config, None);
     let handler = KebabHandler::new(state);
 
     let result = kebab_mcp::tools::schema::handle(