chore: repository scaffolding

The org's standard furniture for a new repo: licence, code of conduct, security
policy, contributing guide, issue and pull-request templates, and the ignore
rules. No sidecar code yet — that arrives as the first pull request, so this
branch exists to open one against.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

.gitignore

@@ -0,0 +1,9 @@
+bin/
+obj/
+*.user
+*.suo
+.vs/
+*.dll
+*.exe
+*.pdb
+*.log