chore: bootstrap repo scaffolding (M0.1)

Adds the §1.2 scaffolding required by IMPLEMENTATION_PLAN.md M0.1:
README, CONTRIBUTING, CODEOWNERS, CHANGELOG, PR + issue templates,
CI workflow, release workflow, LICENSE, commitlint, cliff config,
.editorconfig, .gitignore, .env.example.

Refs: M0.1

.gitignore

@@ -0,0 +1,37 @@
+# OS
+.DS_Store
+Thumbs.db
+
+# Editors
+.vscode/
+.idea/
+*.swp
+*~
+
+# Local secrets
+.env
+.env.local
+.env.*.local
+
+# Build outputs
+dist/
+build/
+out/
+target/
+coverage/
+*.log
+*.tmp
+
+# Node
+node_modules/
+.pnpm-store/
+.next/
+.turbo/
+
+# Go
+*.test
+*.out
+vendor/
+
+# Rust
+**/target/