Contributor guidelines: CONTRIBUTING.md, PR/issue templates, swift-format CI

[gwillish]

Assemble the full contributor experience: finalize CONTRIBUTING.md, add issue and PR templates, and configure swift-format in CI.

Scope

• Finalize and review CONTRIBUTING.md (drafted earlier — may need updates after v1 is functional and lessons learned)
• GitHub issue templates: feature request, bug report
• GitHub PR template: checklist (links issue, tests added, swift-format clean, human author)
• CI: run swift-format lint on PRs via GitHub Actions — fail PR if formatting violations found
• Review and update docs/first-principles.md if any principles need refinement after v1

Depends on

• Phase 4 and Phase 5 complete — want real contributor experience lessons before finalizing

Acceptance criteria

• CONTRIBUTING.md up to date with current conventions and process
• Issue templates present in .github/ISSUE_TEMPLATE/
• PR template present in .github/pull_request_template.md
• CI swift-format check passes on main and fails on a test PR with formatting violations