feat: fix skill spec compliance - migrate to SKILL.md format

[avoidwork]

Summary

Migrate the skills registry from the incompatible skill.yaml/skill.json format to the standard SKILL.md format per the Agent Skills specification (agentskills.io).

Changes

• Discovery: Parse SKILL.md YAML frontmatter instead of skill.yaml/skill.json
• Constraints: Enforce name (1-64 chars, lowercase alphanumeric + hyphens, dir match) and description (1-1024 chars) validation with lenient fallback
• Progressive disclosure: Catalog at startup (name+description), full SKILL.md body loaded via model's file-read on activation
• Sandbox: Replace fork() with spawn() to support .py, .sh, .rb, .ts scripts
• Cross-client scanning: Scan .agents/skills/ alongside skills/, handle name collisions (project overrides user)
• Dropped: inputSchema/outputSchema — not part of the standard
• Updated: openspec/specs/skills-registry/spec.md to reflect SKILL.md requirements

Artifacts

• proposal.md — motivation, changes, capabilities, impact analysis
• design.md — technical decisions, risk mitigation, migration plan
• specs/skills-registry/spec.md — delta spec (removed/added/modified requirements)
• tasks.md — 11 task groups covering all implementation areas

Testing

• Pre-commit hooks passed (lint + coverage)
• Unit test updates needed: new tests for frontmatter parsing, name validation, lenient YAML, interpreter detection, collision handling