docs: add contributor, security, and forge templates

Add CONTRIBUTING/SECURITY/CHANGELOG, Gitea issue and PR templates, and README guidance for Gitea metadata and releases.

Made-with: Cursor

.gitea/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,22 @@
+---
+name: Feature request
+about: Propose new physics, API fields, or GUI workflow improvements
+title: "[feature] "
+labels: enhancement
+---
+
+## Problem / motivation
+
+<!-- What engineering workflow is painful today? -->
+
+## Proposal
+
+<!-- Concrete behavior, API shape, or UI change -->
+
+## Acceptance criteria
+
+<!-- What tests / goldens / docs must change? -->
+
+## References
+
+<!-- Papers, field data, or internal notes -->