Add Zed agent support via ACP proxy

[Krishnachaitanyakc]

Summary

Closes #675

• Implements automatic checkpoint tracking for Zed via ACP (Agent Client Protocol) proxy
• The proxy wraps a real agent (e.g., Claude Code), intercepting tool_call/tool_call_update notifications to create checkpoints automatically — no model cooperation needed
• Architecture: Zed (ACP Client) ←stdio→ git-ai acp-proxy ←stdio→ Real Agent

New files

• src/commands/acp_proxy.rs — Core ACP proxy with Content-Length framed I/O, two-thread forwarding, and background checkpoint spawning
• src/commands/checkpoint_agent/zed_preset.rs — Preset for parsing hook input JSON from the proxy
• src/mdm/agents/zed.rs — ZedInstaller that detects Zed and prints agent_servers setup instructions

Modified files

• src/commands/git_ai_handlers.rs — Wire up acp-proxy command and zed checkpoint preset
• Module declarations in mod.rs files

Test plan

• cargo fmt -- --check passes
• cargo build compiles with no warnings
• 16 unit tests for acp_proxy.rs (message framing, file path extraction, tool call tracking, arg parsing)
• 5 unit tests for zed_preset.rs (pre/post tool use, error cases)
• 4 unit tests for zed.rs (name, id, config hooks flag, install instructions)
• Manual: echo '{"hook_event_name":"PostToolUse","session_id":"s1","cwd":"/tmp","file_paths":["a.rs"]}' | git-ai checkpoint zed --hook-input stdin
• Manual: git-ai acp-proxy -- echo "test" (verifies arg parsing and child spawn)

[dlight]

Thank you for addressing that issue!

I don't know much about how git-ai works, but I'm surprised this approach is needed. I mean, if I use an external agent in Zed like Claude Code, shouldn't Claude Code hooks like pre-edit fire regardless of whether it is being driven by Zed or not? And most external agents Zed could use probably have git-ai support already.

Also I have a question. This proxy won't track edits of the first-party Zed Agent right?
I'm not entirely sure, but I think that in Zed ACP is used for external agents only.

(the Zed Agent is the default agent interface when you open the agent panel, to use external agents you need to select them when creating a new thread).

If this doesn't track edits from the first-party Zed Agent, I think this shouldn't close #675.

[Krishnachaitanyakc]

Good points! You're right that Claude Code's hooks fire independently — running it through the ACP proxy would double-track edits. I'm adding a fix: the proxy sets GIT_AI_ACP_PROXY=1 on the child process, and the Claude preset skips when that env var is present (same pattern as the existing VS Code Copilot/Cursor filters).

The proxy is still useful for agents that don't have native git-ai integration.

Re: first-party Zed Agent — correct, ACP agent_servers is for external agents only. Zed doesn't expose hooks or events for its built-in agent's tool use, so there's no way to intercept those edits currently. I'll file a feature request with Zed for tool-use hooks. That said, this is a Zed-side limitation rather than a gap in this PR's approach.