feat(genomic): add SkillInstaller for fast skill installation via shallow clone

[pyramation]

Summary

Adds SkillInstaller class to genomic that copies .agents/skills/<name>/ directories from git repositories using shallow clones with caching — replacing the slow npx skills add path.

Why: npx skills add downloads the skills npm package on every run, then clones the entire source repo. Two network round-trips per skill. SkillInstaller does a single --depth 1 clone (cached via CacheManager for 7 days), then copies skill directories directly. Subsequent installs from the same source repo are instant.

const installer = new SkillInstaller({ toolName: 'pgpm' });
const result = installer.install(
  [{ source: 'constructive-io/constructive', skills: ['pgpm'] }],
  '/path/to/workspace'
);
// result.installed: ['pgpm']
// result.failed: [] (non-fatal — failures collected, never thrown)

Reuses existing GitCloner (shallow clone + URL normalization) and CacheManager (TTL-based expiration). Groups skills by source repo to avoid duplicate clones.

New exports: SkillInstaller, SkillInstallOptions, SkillInstallResult, SkillInstallFailure.

Companion PR in constructive will wire this into pgpm init as the default skill installer, with --use-skills flag to opt into the slower npx path.

Link to Devin session: https://app.devin.ai/sessions/8a70e4bc95b947faa41db21e02ed818e
Requested by: @pyramation

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring