feat: add validation in abc render subcommand to make `.abc_renamed` or `.abc` reserved

### TL;DR

See context https://github.com/abcxyz/abc/pull/414#discussion_r1483671129

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_

Look at the abc render subcommand code, likely in a cmd/ directory. Find where file naming or validation occurs. The PR #414 discussion linked in the issue provides context on reserving '.abc_renamed' or '.abc'. Determine how to add a validation check to prevent these names. Check for existing tests for the render command to understand the testing pattern.

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