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/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+## Summary
+
+<!-- Briefly describe what this PR does and why. -->
+
+## Checklist
+
+- [ ] Commits follow the [Conventional Commits](https://www.conventionalcommits.org/) spec
+  - `feat:` new feature
+  - `fix:` bug fix
+  - `docs:` documentation only
+  - `chore:` maintenance / tooling
+  - `ci:` CI/CD changes
+  - `refactor:` code change that neither fixes a bug nor adds a feature
+  - `test:` adding or correcting tests
+  - `perf:` performance improvement
+- [ ] `cargo test` passes locally
+- [ ] `cargo clippy --all-targets -- -W clippy::pedantic` passes with no warnings
+- [ ] `cargo fmt --check` passes