Prioritize usable extensions in the chat @ picker

[canvascoding]

The chat @ picker could let catalog entries and connections needing setup obscure usable local extensions. It now loads local results first, refreshes actual connections and package requirements next, and loads catalog offers last below the usable group. Still usable recent references lead the list, and new pages preserve the focused row.

Connected refreshes cover plugins and skills as well as MCP/Composio, including a plugin-only filter. Required MCPs must match the exact template or permitted configuration. A bounded cache retains only immutable package requirements; current source rights and connection policies are checked again. Provider status observations are bounded, scoped and invalidated by the existing Composio lifecycle. Search executes no provider tools or business actions.

Validation completed:

• 40 Composer cases plus Home and stored-session target regressions, including late promotion, changed pins, pagination, retries and stable keyboard focus.
• Real package/SQL readiness cases for required MCPs, mismatched endpoints, optional requirements and cold Composio package promotion. Cold and warm local search add no package source reads; connected preparation reads once per pin.
• Native PostgreSQL ACL/revocation and exact account/package tests; catalog query budgets remain constant at 1/10/50 resources.
• Focused TypeScript across all changed product/test roots and ESLint pass. Existing callback invalidation, metadata, deadlines and E-mail picker regressions pass.

The full production build passes for the product sources in f131930, and the compiled local app is healthy. Final browser acceptance awaits manual navigation of the old test tab out of a data: network-error page; browser policy blocks automated actions from that page. The 300 ms p95 target is not established by these checks. Positive Composio observations use controlled account-status fixtures; this is not a live provider business-action acceptance.

Implementation and validation notes: docs/architecture/canvas-notebook/plugin-tool-gateway/chat-reference-priority-implementation.md.

CodeQL alert #241 was independently reviewed and dismissed as a false positive: the SHA-256 values are private, bounded in-memory change fingerprints, not password verifiers or public credentials. The rule remains enabled; all current checks pass.

Merge note (2026-10-17): The user explicitly requested merging now with the final browser acceptance still outstanding. The browser limitation and the missing UI acceptance proof remain documented; no successful browser acceptance is claimed. All six checks for head 505f3b7 are successful, and there are no unresolved review threads.