feat(skill): add agents skills from ~/.agents/skills

[tusharmath]

Summary

Add support for loading skills from ~/.agents/skills/, enabling interoperability with the common agents ecosystem alongside Forge's existing skill sources.

Context

Forge already supports built-in skills, global skills (~/forge/skills/), and project-local skills (.forge/skills/). This change adds a fourth source — ~/.agents/skills/ — which is the standard directory used by common agent tools. This allows users who maintain skills in that directory to have them automatically available in Forge without any additional configuration.

Changes

• Added agents_skills_path() method to Environment that returns ~/.agents/skills (returns None when home directory is unavailable)
• Updated SkillRepository to load skills from ~/.agents/skills/ after global skills but before project-local skills
• Updated render_skill to support a {{agents_skills_path}} template variable in skill command content
• Updated skill precedence documentation to reflect the new 4-tier order

Key Implementation Details

The new precedence order is: CWD (project-local) > Agents (~/.agents/skills) > Global (~/forge/skills) > Built-in. Skills with duplicate names are resolved by keeping the last occurrence (higher precedence wins). Loading agents skills is gracefully skipped when no home directory is available (None path).

Testing

# Run domain and repo tests
cargo insta test --accept -p forge_domain -p forge_repo

# Manually verify: place a skill in ~/.agents/skills/<skill-name>/SKILL.md
# and confirm it loads in Forge