feat: Cursor CLI ACP provider via Agent Client Protocol

[alecuba16]

Summary

Add Cursor CLI ACP (Agent Client Protocol) provider support, allowing Claurst to connect to Cursor as an LLM backend via the ACP subprocess protocol.

Changes

• New provider (crates/api/src/providers/cursor_acp.rs): CursorAcpProvider implementing LlmProvider — request shaping, response parsing, streaming SSE handling, and tool-call conversion via the ACP wire format.
• Provider registration (registry.rs): CursorAcpProvider registered under the "cursor-acp" key in provider_from_key, provider_from_config, and runtime_provider_for.
• Module exports (providers/mod.rs): pub mod cursor_acp + re-exports of CursorAcpCommand, CursorAcpProvider, ModelCatalog.
• TUI integration (app.rs): provider picker entry, known providers list, and activate_provider match arm for cursor-acp subprocess management.
• Query integration (query/src/lib.rs): "cursor-acp" added to the native providers list.
• Documentation (docs/providers.md): setup instructions, environment variables, and settings.json configuration example.
• Design doc (FEATURE_DESIGN_cursor_cli_acp_support.md): architecture overview and protocol details.

Setup

1. Ensure Cursor CLI is installed and available on $PATH.
2. Add to settings.json:

   {
     "providers": {
       "cursor-acp": {}
     }
   }

Test plan

• cargo check --workspace passes (verified locally)
• Provider appears in the model picker
• Basic prompt round-trip via Cursor ACP subprocess
• Streaming responses render correctly
• Tool-call round-trip works end-to-end

[alecuba16]

Closing in favor of consolidated group branches. The changes from this PR have been merged into a functional group branch.