Add Zed Agent support via MCP server

[Krishnachaitanyakc]

Closes #675

Summary

Adds Zed Agent support for git-ai. Zed's extension API currently lacks tool-use hooks (no pre/post tool use callbacks, no document change events from extensions), so integration uses an MCP server approach.

Components

• git-ai mcp-server — MCP server over stdio with Content-Length framing (MCP/LSP spec). Exposes a git_ai_checkpoint tool that Zed's agent can call before/after file edits
• zed checkpoint preset — Parses hook input from MCP tool calls, creates Human (PreToolUse) and AiAgent (PostToolUse) checkpoints
• Zed installer (src/mdm/agents/zed.rs) — Detects Zed installation (binary + ~/.config/zed/), configures context_servers entry. Respects XDG_CONFIG_HOME
• .zed/rules template (agent-support/zed/rules) — Instructs the agent to call git_ai_checkpoint before/after every file edit

How it works

1. git-ai install-hooks → detects Zed → adds MCP server config to ~/.config/zed/settings.json
2. User copies .zed/rules to their project
3. Zed's agent reads the rules → calls git_ai_checkpoint(event: "PreToolUse") before editing and git_ai_checkpoint(event: "PostToolUse") after
4. MCP server shells out to git-ai checkpoint zed --hook-input stdin
5. Standard git-ai attribution pipeline handles the rest

Design decisions

• Stable session ID: Generated per MCP server instance (not per call), so pre/post checkpoint pairs are properly correlated
• Pipe deadlock prevention: Process spawning starts stdout/stderr readers before writing stdin, per codebase conventions
• Event validation: Preset validates PreToolUse/PostToolUse values

Limitations

• Relies on agent compliance with rules (no enforced hooks — Zed doesn't offer them)
• No transcript capture (Zed doesn't expose conversation data via MCP)
• These are inherent to Zed's current API surface

Test plan

• cargo fmt -- --check — passes
• cargo check — zero warnings
• cargo clippy --lib -- -D warnings — passes
• cargo test zed — all 5 tests pass
• Full test suite — no regressions
• Manual: verify git-ai mcp-server responds to MCP protocol messages
• Manual: verify git-ai install-hooks detects Zed and outputs config instructions