[WIP] Add Cursor ACP provider

[vincenzopalazzo]

Summary

• Add cursor-acp ACP provider that wraps the cursor-agent CLI with acp subcommand
• Uses the Agent Client Protocol to communicate with Cursor, following the same pattern as claude-acp, copilot-acp, amp-acp, and pi-acp
• Binary resolved via cursor-agent (matches the ACP registry entry at agentclientprotocol/registry)
• Registers in the provider registry alongside the existing CLI-based cursor-agent provider
• Adds provider integration test skeleton

Closes #8391

Test plan

• cargo check -p goose --lib passes
• cargo clippy -p goose --lib -- -D warnings passes
• cargo test -p goose --lib -- providers::init passes (registry wiring tests)
• Manual: set GOOSE_PROVIDER: cursor-acp and GOOSE_MODEL: current in config, verify goose connects via cursor-agent acp