feat: add explicit-source agent skills

[lorenjphillips]

Summary

• Adds coval --agent agent skills list and install.
• Requires explicit local --source/COVAL_SKILLS_SOURCE; no repo URL or mutable remote catalog is hardcoded.
• Rejects remote sources with a structured error and tells agents to clone/pin locally.
• Installs by copying a selected local skill directory into explicit --dest/COVAL_SKILLS_DEST.

Testing

• cargo fmt
• cargo clippy --all-targets -- -D warnings
• cargo test

Stacked PR 4 of 5. Base is feat/aci-3-input-json. No version bump or release changes.

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✨ Finishing Touches

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[callumreid]

looks like rust to me!