Implement two-tier skill directory structure and canonical SKILL.md (SK spec)

[ahouseholder]

Summary

Implement the skill infrastructure defined in specs/skills.yaml (prefix SK).
This covers restructuring .agents/skills/ into a two-tier layout (dev/
and ssvc/ subtiers), enforcing the canonical SKILL.md frontmatter format, and
wiring CI validation — all as specified in idea #1151.

Delivered in PR #1142 (commit 36bfa68).

Acceptance Criteria

• AC-1: All skill files live under .agents/skills/dev/ or .agents/skills/ssvc/ (no skills at the flat .agents/skills/ level)
• AC-2: Every SKILL.md has valid YAML front-matter with non-empty name and description fields
• AC-3: .github/workflows/validate_skills.yml passes on all existing SKILL.md files
• AC-4: Expand evaluate-decision-point from WIP stub to complete skill → tracked in Add LLM-based decision point evaluator skill and supporting library tooling #1152

Reference

Spec: specs/skills.yaml
Notes: notes/skills.md
Idea: #1151

[ahouseholder]

AC-1, AC-2, and AC-3 all delivered in PR #1142 (commit 36bfa68). AC-4 is out of scope here — tracked in #1152.