Path traversal via unvalidated entityId from SQS message

## Summary
When the avatar comes inline in the SQS message, `consumer.ts:77-88` uses the message's `entity` without fetching the catalyst (so the id is never validated), and `message-validator.ts:50-58` only checks that `entityId` exists. `adapters/godot.ts:152-153` then interpolates `entity` into `path.join(outputPath, `${entity}_body.png`)`. An `entityId` like `../../..` writes the generated PNG outside the intended directory.

## Impact
Conditional on who can publish to the SQS queue (IAM). Out-of-directory file write.

## Fix
Validate `entityId` against the CID/hash format in `message-validator.ts` before using it in any path.