feat: add DefaultEnv config field for enable commands

[damianoneill]

What does this PR do?

Add a DefaultEnv map[string]string field to Config that is automatically merged into the server env when enable writes a config for any editor (Claude, VSCode, Cursor). User-provided --env values take precedence on conflict.

This solves the problem where editors launch MCP server subprocesses with a minimal PATH that cannot find executables managed by mise, asdf, homebrew, nix, or installed to non-standard paths.

Consumers can now capture PATH (or any other env vars) at config time:

&ophis.Config{
    DefaultEnv: map[string]string{
        "PATH": os.Getenv("PATH"),
    },
}

Backwards compatible: nil DefaultEnv preserves current behavior.

Related Issues

Closes #39 (follow-up)

Checklist

• Tests added or updated
• Docs added or updated

[github-actions]

Review Summary

This PR successfully adds the DefaultEnv feature to allow configuring default environment variables for MCP servers. The implementation is solid and well-tested.

Implementation Quality

✅ Clean design: The DefaultEnv field is added to the Config struct with clear documentation
✅ Backwards compatible: nil DefaultEnv preserves current behavior
✅ Consistent implementation: The merge logic is identical across all three editors (Claude, VSCode, Cursor)
✅ User precedence: User-provided --env values correctly override defaults
✅ Comprehensive tests: Tests cover all key scenarios:

• Default env written to config
• User override behavior
• nil/empty DefaultEnv handling
• Mixed default + user env

Code Review

Merge Logic (claude/enable.go:66-77, vscode/enable.go:71-81, cursor/enable.go:68-78):
The implementation correctly:

1. Creates a new map to avoid mutating the config
2. Copies default env first
3. Overlays user env (giving user values precedence)
4. Only sets server.Env if non-empty

Threading: The defaultEnv is properly threaded through:

• Config → root.Command() → editor commands → enableCommand() → merge logic

Documentation: README.md and docs/config.md provide clear examples and explain the PATH problem this solves.

No Issues Found

No bugs, security issues, or significant design problems identified. This is ready to merge.