feat: Agent Skills spec compliance

[caohy1988]

Summary

• Frontmatter validation: Add field validators for name (kebab-case, max 64 chars), description (non-empty, max 1024 chars), compatibility (max 500 chars); add allowed-tools YAML alias with populate_by_name=True; set extra='allow'
• Skill source_path: Track the filesystem path to the loaded SKILL.md; emit <location> tag in XML prompt when available
• Utils improvements: Extract _parse_skill_md() helper; use model_validate() for alias support; enforce name-directory matching per spec; add validate_skill_dir() and read_skill_properties() public functions
• SkillToolset enhancements: Add scripts/ resource loading via load_skill_resource; auto-inject DEFAULT_SKILL_SYSTEM_INSTRUCTION (with inject_instruction=False opt-out); duplicate skill name check; _list_skills() returns Skill objects for richer XML output
• Sample agent fix: Remove manual instruction= (auto-injected now); rename weather_skill/ → weather-skill/ to match spec naming requirements

Closes gaps against the Agent Skills spec.

Test plan

• All 55 skills + skill_toolset tests pass (pytest tests/unittests/skills/ tests/unittests/tools/test_skill_toolset.py)
• Full unit test suite passes (4265 passed, 1 skipped — skip is pre-existing)
• Autoformatter passes (./autoformat.sh)
• Sample agent imports successfully

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