chore: prepare CronSnap for OSS contributor adoption

[sidmohan0]

Summary

This PR applies a single OSS-readiness sweep requested by maintainers. It makes CronSnap easier and safer for broader open-source adoption by adding governance, contributor UX, installation/release documentation, and CI hardening without changing runtime behavior.

Changes

• Added repository policy and governance files: LICENSE, SECURITY.md, CODE_OF_CONDUCT.md, CONTRIBUTING.md.
• Added GitHub collaboration scaffolding:
  • .github/ISSUE_TEMPLATE/bug_report.md
  • .github/ISSUE_TEMPLATE/feature_request.md
  • .github/PULL_REQUEST_TEMPLATE.md
  • .github/CODEOWNERS
• Updated README.md with:
  • badges,
  • quick install matrix,
  • links to license/governance docs,
  • command list aligned with current --mode flags.
• Added docs/RELEASING.md for release workflow and artifact expectations.
• Added .github/workflows/ci.yml with:
  • go test ./...
  • shellcheck scripts/adapters/cronsnap-git-wrapper.sh
  • existing docs/spec/spike/runbook lint gates
• Cleaned temporary repository artifact from tracking (.tmp-ab-spike/a.txt) and updated .gitignore.
• Updated repo-level metadata (via GitHub): description and topics (policy/events-focused tags).

Validation

• go test ./... ✅
• bash scripts/ci/cs-plans-lint.sh ✅
• bash scripts/ci/cs-spikes-lint.sh ✅
• bash scripts/ci/cs-specs-lint.sh ✅
• bash scripts/ci/cs-docs-lint.sh ⚠️ (existing warning in docs/design-docs/core-beliefs.md about seed markers)
• bash scripts/ci/cs-runbooks-lint.sh ✅
• shellcheck scripts/adapters/cronsnap-git-wrapper.sh (not installed in local environment; CI installs it in workflow)

Notes

No functional runtime behavior changed.