Wave 5: Schemas command, sync command, network policy, test fixtures (bd-x15, bd-3f4, bd-1cv, bd-lx6)

- Implement schemas command with list/show modes, regex filtering, ref expansion
- Implement sync command with conditional fetch, content hash diffing, dry-run
- Add NetworkPolicy enum (Auto/Offline/OnlineOnly) with env var + CLI flag resolution
- Integrate network policy into AsyncHttpClient and fetch command
- Create test fixtures (petstore.json/yaml, minimal.json) and integration test helpers
- Fix clippy lints: derivable_impls, len_zero, borrow-after-move, deprecated API
- 192 tests passing (179 unit + 13 integration), all quality gates green

src/core/mod.rs

@@ -2,6 +2,7 @@ pub mod cache;
 pub mod config;
 pub mod http;
 pub mod indexer;
+pub mod network;
 pub mod refs;
 pub mod search;
 pub mod spec;