Codex: validate project_doc_fallback_filenames semantics in .codex/config.toml

[avifenesh]

Summary

Codex AGENTS docs now document project_doc_fallback_filenames as an active instruction-discovery mechanism with explicit precedence behavior.

Source: https://developers.openai.com/codex/guides/agents-md/

agnix currently recognizes the key as known but does not validate semantic correctness for this field.

Current Gap

• .codex/config.toml parsing/rules validate approvalMode, fullAutoErrorMode, project_doc_max_bytes, and unknown keys.
• No dedicated validation for project_doc_fallback_filenames shape/content.

Proposed Work

• Add Codex rule(s) for project_doc_fallback_filenames:
  • must be an array
  • items must be non-empty strings
  • reject duplicates (or warn)
  • optionally flag suspicious names (path separators, absolute paths)
• Update knowledge-base/rules.json and knowledge-base/VALIDATION-RULES.md in parity.
• Add unit/integration tests for valid + invalid cases.

Acceptance Criteria

• Invalid fallback-filename configs produce deterministic diagnostics.
• Rules/docs parity tests pass.
• New tests cover parsing + lint diagnostics.