feat: add create_skill tool for spec-compliant skill scaffolding

[avoidwork]

Description

Add a create_skill LangChain tool that creates Agent Skills spec-compliant skills from agent invocations. The tool validates all metadata against the spec (name constraints, description length, permission scopes), creates the skill directory structure with SKILL.md, and optionally scaffolds a scripts/ directory. Agents can then use write_file to populate instructions or create scripts.

• Add createSkillImpl to src/tools/skills.js with full Agent Skills spec validation
• Register tool in src/tools/index.js gated by filesystem:write permission
• Add createCreateSkillTool factory for runtime options binding
• Create tests/unit/skills.test.js with 38 tests covering name/desc/permission validation,
  duplicate detection, directory creation, YAML frontmatter generation, scripts scaffolding,
  and registry registration
• Fix outdated docs referencing skill.yaml/skill.json — now fully documents SKILL.md format
• Update README.md Extending Skills section with spec-compliant manual and programmatic creation guides
• Fix src/registry/ → src/skills/ in README directory structure (actual source layout)
• Add create_skill to all Permission Gating and Tool Configuration tables across docs

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

Added tests/unit/skills.test.js with 38 tests covering:

• Tool registration (exports, type, name)
• Name validation (uppercase, leading/trailing hyphens, consecutive hyphens, underscores, empty, numeric+alpha)
• Description validation (empty, whitespace-only, over 1024 chars, valid, minimal)
• Permission validation (valid, invalid, mixed valid/invalid, empty array, undefined)
• Duplicate detection (rejects when already registered in registry)
• Skill directory creation (directory + SKILL.md creation, valid YAML frontmatter, optional license/compatibility/metadata fields, registry registration, warnings)
• Scripts scaffolding (creates with scaffoldScripts=true, skips with false/default)
• Return value structure (paths array on success, errors array on failure)
• Catalog prompt generation (empty/skill entries formatting)

All 38 tests pass. Full test suite: 1054 tests pass.

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied