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

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+This repository is under active development. Security fixes land on the default branch (`main`) first.
+
+## Reporting a vulnerability
+
+Please email the repository maintainers with:
+
+- a short description of the issue
+- steps to reproduce (if applicable)
+- affected component (`solver-api`, `gui-ts`, `solver-c`, Docker, etc.)
+
+Do not open a public issue for undisclosed vulnerabilities.
+
+## Scope notes
+
+- The local API is intended for **trusted development networks**. Do not expose it to the public internet without hardening (TLS, auth, rate limits, reverse proxy).
+- Treat uploaded XML as untrusted input at API boundaries.