feat: v0.3.0a — add api command, pr diff/comment/review, structured errors

New commands:
- fgj api: raw REST API passthrough with field inference and path interpolation
- fgj pr diff: view PR diffs with color, --name-only, --stat
- fgj pr comment: add comments to pull requests
- fgj pr review: approve, request changes, or comment on PRs

Agentic enhancements:
- --json-errors flag for structured JSON error output on stderr
- APIError type wrapping HTTP status codes for machine consumption
- Error codes: auth_required, not_found, api_error, invalid_input, etc.

Docs updated for forgejo.zerova.net/sid/fgj-sid fork.

cmd/errors.go

@@ -5,7 +5,7 @@ import (
 	"errors"
 	"os"
 
-	"codeberg.org/romaintb/fgj/internal/api"
+	"forgejo.zerova.net/sid/fgj-sid/internal/api"
 )
 
 // Error codes for structured error output.