feat: add multi-vendor agent skill distribution

[keepithuman]

Description

Adds a source/generated multi-vendor distribution model for the Itential builder skills. The canonical source remains AGENTS.md plus skills/*, while vendor-specific UX surfaces are generated for Claude, Codex, Cursor, and GitHub Copilot.

Type of Change

• Bug fix
• New feature
• Documentation
• Refactor
• Chore

Changes Made

• Adds canonical skills/ source directory for domain skill content.
• Adds generated Claude command wrappers under .claude/commands/.
• Adds GitHub Copilot repo instructions and prompt wrappers.
• Adds Cursor routing rules.
• Adds a single installable Codex meta-skill bundle under codex/itential-builder-skills/.
• Adds repo-native governance in docs/constitution.md.
• Adds org/team/developer customization layers.
• Adds generation and validation scripts for vendor artifacts.
• Documents vendor install paths and the source/generated architecture.

Testing

• Ran scripts/generate-vendor-wrappers.sh
• Ran scripts/check-vendor-skills.sh
• Ran scripts/check-generated.sh
• Ran scripts/use-skill builder-agent
• Ran git diff --cached --check before commit
• Verified no .env, .auth.json, key, credential, or secret files were staged

Related Issues

None.

Checklist

• Code follows the project's style guidelines
• Self-review of code has been performed
• Code has been commented where necessary
• Tested with relevant repository scripts
• Commits follow conventional format (type: subject)
• No secrets or credentials committed
• Documentation has been updated accordingly
• PR has been labeled appropriately (enhancement, documentation, chore)