feat(pgpm): use built-in SkillInstaller for fast skill installation

[pyramation]

Summary

Replaces npx skills add with genomic's SkillInstaller as the default skill installer in pgpm init. The old npx path is available via --use-skills flag.

Why: npx skills add was slow (~30s+) — it downloads the skills npm package, clones the entire source repo, then copies files. SkillInstaller does a single shallow clone (cached for 7 days) and copies .agents/skills/<name>/ directly. Subsequent installs from the same source are instant.

Changes:

installSkills() now dispatches to installSkillsBuiltin() (default) or installSkillsViaNpx() (with --use-skills):

- function installSkills(skills, cwd)
+ function installSkills(skills, cwd, useNpxSkills)
+   if (PGPM_SKIP_SKILL_INSTALL) → return (test mode)
+   if (useNpxSkills) → installSkillsViaNpx()  // old npx path
+   else              → installSkillsBuiltin()  // new SkillInstaller

--use-skills flag threaded through all init paths: handleInit → handleWorkspaceInit / handleModuleInit / handleBoilerplateInit.

Re-exports SkillInstaller, SkillInstallOptions, SkillInstallResult, SkillInstallFailure from @pgpmjs/core for downstream consumers.

PGPM_SKIP_SKILL_INSTALL env var added for deterministic testing (same pattern as PGPM_SKIP_UPDATE_CHECK).

Depends on: dev-utils#88 (merged) — adds SkillInstaller to genomic. Requires genomic@5.6.0.

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

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