chore: add github project framework

[christian-deleon]

Summary

• Add AGENTS.md as canonical agent intelligence file, replace CLAUDE.md with symlink
• Add CONTRIBUTING.md with development setup, commit conventions, and PR process
• Add agent skills (.agents/skills/) for validate, commit, and PR workflows
• Add GitHub issue templates (bug report, feature request) and PR template
• Add CI workflow with format check, unit tests, and PR title validation
• Add lint and check recipes to justfile
• Update .claude/settings.json with allowed commands for agent skills

Test Plan

• just lint passes
• just test-unit passes (CI will verify)
• CI workflow runs on this PR (Format Check, Unit Tests, PR Title)