Add installer quickstart and maintainer templates

[TheRobertRay]

Summary

• Add a skill installer and install/use guide so maintainers can adopt the kit directly.
• Add GitHub issue and PR templates for workflow triage, validation, release notes, and risk tracking.
• Strengthen validation with installer smoke tests and install-guide skill coverage checks.

Validation

• bash scripts/validate.sh
• bash -n scripts/install-skills.sh
• bash -n scripts/validate.sh
• git diff --check
• invalid skill-name smoke test
• milestone reviewer pass with no blockers

Closes #6