feat(pgpm): install skills from .boilerplate.json after scaffold

[pyramation]

Summary

Adds automatic skill installation to pgpm init workspace and pgpm init (module) flows. Templates can now declare agent skills in .boilerplate.json:

{
  "skills": [
    { "source": "constructive-io/constructive", "skills": ["pgpm"] },
    { "source": "constructive-io/constructive-skills", "skills": ["constructive-data-modeling"] }
  ]
}

After scaffoldTemplate completes, inspectTemplate reads the config and runs npx --yes skills add <source> --skill <name> --yes for each entry. Spawns with stdio: ['pipe', 'inherit', 'inherit'] to avoid readline conflicts with Inquirerer while still showing progress output.

Non-fatal on failure — prints the manual commands the user can run themselves:

⚠️  Some skills could not be installed automatically.
Run the following commands manually:

  npx skills add https://github.com/constructive-io/constructive --skill pgpm

New types in @pgpmjs/core:

interface BoilerplateSkill {
  source: string;   // "org/repo" or full URL
  skills: string[]; // skill names to install
}

Companion PR: constructive-io/pgpm-boilerplates#32 adds the skills field to the workspace template's .boilerplate.json.

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

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