feat: Add Cursor CLI ACP provider (cursor-acp)

Summary

Implemented a separate native cursor-acp provider that uses Cursor's Agent Client Protocol over stdio, distinct from the existing direct HTTPS --provider cursor.

Branch & Commit

• Branch: feature/cursor-acp

Key Changes

• New crate jcode-provider-cursor-acp-runtime with controlled agent acp subprocess lifecycle
• ACP event mapping: text, thinking, tool calls, permissions, usage, images
• Model discovery only from ACP session/new + config updates (no static list)
• Deterministic resolution: exact opaque IDs, unique bare variants, explicit errors for ambiguous/unsupported
• Config via env: JCODE_CURSOR_ACP_PATH, JCODE_CURSOR_ACP_ARGS, JCODE_CURSOR_ACP_MODEL, JCODE_CURSOR_ACP_PERMISSION
• CLI: --provider cursor-acp with parser/registration tests
• Auth-test: skips tool smoke (Cursor owns tool execution)
• 10 focused unit/integration tests including fake ACP subprocess
• Docs: docs/CURSOR_ACP.md
• Final selfdev TUI build passes

Deliberate Separations

• Direct cursor provider unchanged (HTTPS-based)
• No static model fallback
• Fail-closed permission handling (default reject_once)

Testing

cargo test -p jcode-provider-cursor-acp-runtime      # 10 tests pass
cargo test -p jcode --lib external_provider_runtimes_register_and_instantiate
cargo test -p jcode --lib test_provider_choice_aliases_parse
cargo test -p jcode --lib skips_cursor_acp_tool_smoke
cargo fmt --check && cargo check -p jcode --bin jcode
selfdev build target=tui                              # builds successfully

Start by reviewing the feature/cursor-acp branch and the jcode-provider-cursor-acp-runtime crate, then run the listed cargo tests and formatting checks. Done means the cursor-acp provider is registered, its focused tests pass, the CLI parser and auth behavior are covered, and docs/CURSOR_ACP.md is present.