feat(skills): support repo and external commands

[shiny-code-bot]

Summary

• Add commands[].source for explicit skill, repo, and external command metadata.
• Preserve strict resource_path validation for default source: skill bundled helper commands.
• Render repo/external commands without pretending they are skill-bundled helper scripts.
• Migrate .codex/skills/remote-tests and .codex/skills/test-tui onto structured repo/external command metadata.

Design Notes

Refs #260.

This keeps the existing bundled helper contract intact:

• source: skill is the default for backwards compatibility.
• source: skill still requires resource_path, a matching resources[] entry, and an existing helper file.
• source: repo is allowed only for repo-scoped skills and must not declare resource_path.
• source: external must not declare resource_path and does not perform filesystem checks.

The schema is intentionally small. It does not add command variants, option metadata, network/capability hints, or shared skill-library migration.

Agent Review

Design reviewed by Opus, Antigravity/Gemini-style, and GPT agents before implementation. All agreed on the key invariant: do not relax bundled resource_path validation. The final implementation uses one command list with an explicit source discriminator rather than fake resources for repo/external workflows.

Validation

• cargo test -p code-core skills::
• ./build-fast.sh