feat(catalog): add README category index generation

[tianlinyang77]

Summary

Generate a category index in README.md so the catalog stays navigable as skills are added. Today the README lists skills by product only; once the catalog grows past roughly ten skills, a reader cannot tell which skill applies to their task without opening each one.

category is already required in components.d/*.yml and already drives skills.sh.json groupings, so this change reuses existing metadata rather than introducing a new field.

What changed

• scripts/generate_catalog.py renders a <!-- categories:start --> / <!-- categories:end --> section containing a skill/category count summary and one table per category.
• README.md gains the Skills by category section and its markers.
• components.d/README.md documents that category drives the generated index, and asks contributors to reuse an existing category name when one fits.
• tests/test_skillhub.py covers grouping, deterministic ordering, singular/plural labels, table escaping, and the missing-marker error.

Output is deterministic: categories sort by lowercased name, skills sort by catalog_dir, and pipes and newlines in descriptions are escaped. generate_catalog.py --check therefore still detects drift.

With the current single skill the section renders as:

1 skill across 1 category.

### Developer Tools

| Skill | Product | Description |
|---|---|---|
| [`skillhub-contributor`](skills/skillhub-contributor) | SkillHub | Create, review, and onboard portable Agent Skills into Hygon SkillHub. ... |

Publication checks

• Every registered source-of-truth repository is owned by HYGON-AI.
• No skill content, admission rules, or synchronization behavior is changed.
• Third-party attribution, license, and NOTICE requirements are preserved.
• The source license permits redistribution.
• Executable scripts and dependencies were reviewed.
• No credentials, private endpoints, customer data, or unpublished information are included.
• Local validation and generated-catalog checks pass.
• Commits include DCO sign-off.

Validation

• python -m unittest discover -s tests — 8 tests pass
• python scripts/validate_skills.py
• python scripts/generate_catalog.py --check
• HYGON submission compliance check