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

CHANGELOG.md

@@ -0,0 +1,5 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+This changelog is generated by [cocogitto](https://github.com/oknozor/cocogitto).