[plan] Document engine-specific feature compatibility in schema

[github-actions]

Objective

Add engine compatibility metadata to schema $comment fields for features that only work on specific engines.

Context

Multiple features are engine-specific (firewall only works on copilot and codex, sandbox only on copilot), but the schema provides no compatibility information. Users configure unsupported features and only discover the incompatibility at runtime or through obscure errors.

Engine-Locked Features to Document

Feature	Supported Engines	Code Reference
firewall	copilot, codex	pkg/workflow/firewall.go:89,105
sandbox	copilot	pkg/workflow/firewall.go:135
HTTP transport variations	Engine-specific	pkg/workflow/engine_validation.go
max-turns behavior	Engine-specific	pkg/workflow/agent_validation.go:143

Approach

Add $comment metadata to each feature's schema definition:

{
  "firewall": {
    "type": "object",
    "$comment": "Engine compatibility: copilot, codex only. Not supported on claude or custom engines. See pkg/workflow/firewall.go:89",
    "properties": { ... }
  }
}

For the engine field itself, add a compatibility matrix:

{
  "engine": {
    "type": "string",
    "enum": ["copilot", "claude", "codex", "custom"],
    "$comment": "Feature compatibility by engine: firewall (copilot, codex), sandbox (copilot only), http-transport (varies)"
  }
}

Files to Modify

• pkg/parser/schemas/frontmatter.json - Add $comment fields
• After changes, run make build to rebuild with embedded schema

Acceptance Criteria

• All engine-specific features have $comment compatibility notes
• Comments reference code locations for verification
• Engine field has feature compatibility matrix
• Documentation updated to reference schema comments

Testing

Review schema with IDE that displays $comment fields to verify visibility.
Related to #7575

AI generated by Plan Command for discussion #7569