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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## What does this PR do?
+
+<!-- One or two sentences. -->
+
+## Why?
+
+<!-- Link to the issue this addresses, or explain the motivation if there isn't one. -->
+
+Closes #
+
+## Changes
+
+<!-- Bullet list of the meaningful changes. -->
+
+-
+
+## Testing
+
+<!-- How did you verify this works? New tests added? Manual test steps? -->
+
+## Checklist
+
+- [ ] Targets the `dev` branch (not `main`)
+- [ ] `uv run pytest` passes
+- [ ] `uv run ruff check` passes
+- [ ] `CHANGELOG.md` `[Unreleased]` section updated