harness-providers: models.dev catalog + cost display wiring (M3)

- modelsdev.rs: ModelCatalog parses models.dev api.json → ModelInfo keyed by
  (provider, model); 24h file cache at ~/.cache/ai-harness/models.json with a
  baked assets/models-snapshot.json fallback so cost/limits work offline.
  load_cached_or_baked (no network) + refresh/refresh_default_cache (background).
- App: loads the catalog at init (cached-or-baked, never blocks), warms the cache
  in a background task, and passes the session model's pricing into RunConfig.cost
  so session cost accrues for real.
- TUI: AppState tracks session_cost/session_tokens from Session events; status bar
  shows "<tokens> · $<cost>". Snapshots updated.
- 7 modelsdev tests (parse, defaults, unknown-model, baked snapshot, cache TTL,
  cached-vs-baked load).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/harness-tui/src/input.rs

@@ -187,6 +187,8 @@ pub async fn apply_action(action: InputAction, state: &mut AppState, engine: &En
                 state.session_id = Some(id);
                 state.messages.clear();
                 state.scroll_offset = 0;
+                state.session_cost = 0.0;
+                state.session_tokens = 0;
                 state.dirty = true;
             }
             Err(e) => tracing::error!(error = %e, "failed to create session"),