ci: overhaul CI/CD pipeline and add tooling configs

- Add ci.yml: test matrix (ubuntu/macos/windows), lint, conventional
  commits check (cocogitto), cargo-deny, MSRV, and semver jobs
- Add release.yml: tag-triggered publish to crates.io + GitHub release
- Remove stale rust.yml and rust-clippy.yml workflows
- Add deny.toml for cargo-deny license/advisory/ban/source checks
- Add cog.toml for cocogitto conventional commits and changelog generation
- Add Justfile with test, check, fmt, commits, release, push-tag targets
- Add CHANGELOG.md seed file
- Add PULL_REQUEST_TEMPLATE.md with conventional commits checklist
- Update dependabot.yml: weekly schedule with grouped patch/minor updates

.github/dependabot.yml

@@ -3,9 +3,23 @@ updates:
   - package-ecosystem: "cargo"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
+    groups:
+      patch-updates:
+        update-types:
+          - "patch"
+      minor-updates:
+        update-types:
+          - "minor"
 
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
+    groups:
+      patch-updates:
+        update-types:
+          - "patch"
+      minor-updates:
+        update-types:
+          - "minor"