fix: aidd-custom/index.md omits config.yml from directory listing

[ianwhitedeveloper]

Problem

aidd-custom/index.md does not include an entry for config.yml, despite the file being present in the aidd-custom/ directory.

Per AGENTS.md, agents rely on index.md files to understand a directory's contents without reading every file (progressive discovery). A missing config.yml entry means agents following this protocol will be unaware of the project-level configuration file and miss settings like e2eBeforeCommit and agent-config.

This was surfaced as a medium-severity finding in the paralleldrive/mono PR #13 review.

Expected Behaviour

aidd-custom/index.md should list config.yml with a brief description of its purpose so agents can discover it via the index.

Example entry:

### AIDD Custom Configuration

**File:** `config.yml`

Controls AIDD framework behaviour for this project (e2e test gating, agent selection).

Notes

• The index.md files are auto-generated from frontmatter by the pre-commit hook (npx aidd --index). The fix may require adding appropriate frontmatter to config.yml (or an adjacent descriptor) so the generator picks it up, rather than editing the index directly.
• Verify the generation logic handles non-markdown files like .yml.