feat(acp): add editor protocol server

[yan-ad]

Summary

• Add crabcode acp [--cwd PATH], a stdio Agent Client Protocol server backed by the Rust ACP SDK.
• Implement persisted ACP session lifecycle: create, list, load, resume, close, fork, prompt streaming, cancellation, mode selection, tool updates, and permission forwarding.
• Make configuration and history initialization workspace-aware for ACP sessions.
• Document Zed setup and add an ACP capability matrix with supported behavior, limitations, and follow-up work.

Validation

• cargo fmt --check
• cargo test acp::service::tests --bin crabcode
• cargo test parses_acp_command_with_workspace --bin crabcode
• cargo test config::configuration --bin crabcode
• git diff --check

Notes

• The capability matrix intentionally documents unsupported image/audio ACP prompts, client-supplied MCP overlays, terminal embedding, model/effort selection, and authoritative usage updates instead of advertising them.

[Blankeos]

Was thinking of this for a while now. Thanks for adding! Will review asap