🚑 docs + cleanup: ARCHITECTURE.md kebab-mcp + state.rs stale comment + schema test cap-flag (fb-30)

Final review fixes:

- docs/ARCHITECTURE.md: add kebab-mcp to UI subgraph + directory tree
  (CLAUDE.md "add new crate" rule required this; missed in Task 12 doc sync).
- state.rs: replace forward-reference Task 10 comment with current-state
  doc (config_path now wired by Task 10 commit 4a30959).
- tools_call_schema.rs: assert capabilities.mcp_server == true (already
  pinned in schema_report + cli_schema, this closes the gap in mcp's own
  test).

Version bump 0.3.0 → 0.4.0 deferred to separate `chore/bump-v0.4.0` PR
mirroring fb-27 precedent (commit 73f5d73 / PR #105).

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

crates/kebab-mcp/tests/tools_call_schema.rs

@@ -67,4 +67,9 @@ async fn schema_tool_returns_schema_v1_json() {
         Some("schema.v1"),
         "unexpected schema_version in: {v}"
     );
+    assert_eq!(
+        v.get("capabilities").and_then(|c| c.get("mcp_server")).and_then(|b| b.as_bool()),
+        Some(true),
+        "mcp_server capability flag should be true after fb-30",
+    );
 }