[Feature]: Support ~/.agents/skills as alternative skills directory

[dpshde]

Problem

Currently, Forge only supports skills from two hardcoded locations:

• Global: ~/forge/skills/
• Project-local: .forge/skills/

Users who already have skills in ~/.agents/skills (used by other AI coding tools like Codex and Amp Code) cannot reuse them with Forge without manually symlinking or copying the directory.

Solution

Add support for ~/.agents/skills as an additional skills search path, either as:

1. Automatic fallback: Check ~/.agents/skills if ~/forge/skills doesn't exist or is empty
2. Environment variable: Support FORGE_SKILLS_PATH to override the default location
3. Multiple paths: Allow a search path list (similar to $PATH)

Prior Art

• OpenAI Codex: Uses ~/.codex/skills/ but also respects XDG conventions
• Amp Code: Already supports ~/.agents/skills/ as a shared location for AI agent skills

Benefits

• Interoperability: Skills created for one AI tool become available to others
• User convenience: No need to maintain duplicate skills across different tools
• Ecosystem growth: Encourages a shared skill marketplace/repository

Proposed Implementation

Modify the skill loading logic in crates/forge_repo/src/skill.rs to:

1. Check ~/.agents/skills/ as a tertiary fallback after ~/forge/skills/
2. Or introduce FORGE_SKILLS_PATH environment variable for explicit override

Related Issues

• [Feature]: Switch to ~/.config/forge from ~/forge (XDG compliant) #2451: [Feature]: Switch to ~/.config/forge from ~/forge (XDG compliant)