Add community scaffolding: CONTRIBUTING, SECURITY, issue templates, PR template

- CONTRIBUTING.md: dev-branch workflow, uv setup, test/lint commands
- SECURITY.md: private disclosure to holden@arch.fyi
- .github/ISSUE_TEMPLATE/bug_report.yml: structured form with image tag, versions, logs
- .github/ISSUE_TEMPLATE/feature_request.yml: problem/solution/alternatives form
- .github/PULL_REQUEST_TEMPLATE.md: checklist enforcing dev branch + passing CI
- pyproject.toml: add Changelog and Documentation URLs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SECURITY.md

@@ -0,0 +1,17 @@
+# Security Policy
+
+## Supported Versions
+
+Only the latest release is supported with security fixes.
+
+## Reporting a Vulnerability
+
+Please do **not** open a public GitHub issue for security vulnerabilities.
+
+Email **holden@arch.fyi** with:
+
+- A description of the vulnerability and its potential impact
+- Steps to reproduce or a proof of concept
+- Any suggested fix, if you have one
+
+You will receive an acknowledgement within 48 hours. If the vulnerability is confirmed, a fix will be released as soon as possible and you will be credited in the changelog unless you prefer otherwise.