feat: skill validate and fix CLI commands

[christso]

Summary

Add `allagents skill validate` and `allagents skill fix` commands for CI/CD pipelines in skills repositories.

Motivation

Research into WiseTechGlobal/Tools.WtgCopilotSkills showed their sync script validates YAML frontmatter fields, strips invalid fields, converts arrays to comma-separated strings, and removes BOM characters. This validation belongs in CI/CD for skills repos, not at sync time.

Proposed Solution

`allagents skill validate [path]`

• Validate YAML frontmatter fields against known schema
• Check for invalid/unsupported fields
• Verify required fields (`name`, `description`) are present
• Report BOM characters or encoding issues
• Exit non-zero on errors (CI-friendly)
• Default path: current directory (scans recursively for `SKILL.md` files)

`allagents skill fix [path]`

• Auto-fix common issues:
  • Strip BOM characters
  • Remove invalid frontmatter fields
  • Normalize field formats (e.g., array → comma-separated string where required)
• `--dry-run` mode to preview changes without writing

Usage

```bash

In CI pipeline

allagents skill validate ./skills/

Local development

allagents skill fix ./skills/ --dry-run
allagents skill fix ./skills/
```

Implementation Notes

• Existing validation logic lives in `src/validators/skill.ts`:
  • `validateSkill()` — validates a single skill file
  • `validateAllSkills()` — validates all skills in a directory
  • `parseSkillMetadata()` — parses YAML frontmatter from skill content
• Skill metadata parsing also in `src/core/skills.ts` and `src/core/repo-skills.ts`
• CLI commands are in `src/cli/commands/` — existing `plugin-skills.ts` handles skill listing
• Add new `skill.ts` command file (or extend `plugin-skills.ts`)
• The `fix` command should reuse the same validation logic but write corrections back to files
• Consider structured output (`--json`) for CI integration

Acceptance Criteria

• `allagents skill validate ./path` scans for `SKILL.md` files and reports validation errors
• Exits with non-zero code when errors found (CI-friendly)
• Validates: required fields present, no unknown fields, correct field types, no BOM
• `allagents skill fix ./path` auto-corrects fixable issues
• `allagents skill fix --dry-run` previews changes without writing
• Clear error output showing file path, line/field, and issue description
• Works on both flat (`skills/foo.md`) and directory (`skills/foo/SKILL.md`) layouts

Context

This is a CI/CD concern for skills repo maintainers, not a sync-time concern. Keeps AllAgents sync fast and pushes validation left to where skills are authored.