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.
• Support PNG, JPEG, GIF, and WebP ACP image attachments when the selected model accepts image input; audio input remains unsupported.
• Expose selectable Crabcode models to ACP clients with validated, session-local provider/model changes.
• Expose model-supported reasoning-effort selectors through ACP session configuration options.
• Emit estimated ACP context-window usage when the selected model provides a context limit.
• Add crabcode models [provider] for non-interactive model discovery using the same credentials, custom providers, free models, runtime models, and effective catalog as Crabcode.
• 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.

Editor Validation

The crabcode acp command was tested successfully with Zed IDE:

• Zed version: 1.13.1
• Zed commit: 00bd72e7838f4b875a913cd112b47a0ebe1ca62b
• Full version: 1.13.1+stable.332.00bd72e7838f4b875a913cd112b47a0ebe1ca62b

Validation

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

Notes

• ACP model choices use canonical provider/model identifiers and remain session-local.
• ACP reasoning-effort options are available only when model catalog metadata provides supported effort values.
• ACP usage currently reports estimated context tokens and the known context-window size; provider-authoritative token and cost accounting is still pending.
• Remaining ACP limitations are client-provided MCP servers, ACP terminal embedding, audio prompts, and authoritative usage/cost reporting.

[Blankeos]

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

[yan-ad]

@Blankeos this PR is ready for review. I have tested the ACP integration end-to-end in my local Zed setup, including image prompts, slash-command discovery/execution, workspace skills, and configured/client-provided MCP servers. Everything is working locally, and the ACP test suite plus cargo check pass.