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

cog.toml

@@ -0,0 +1,32 @@
+# Cocogitto configuration
+# https://docs.cocogitto.io/config/
+
+ignore_merge_commits = true
+tag_prefix = "v"
+
+# Update Cargo.toml version before committing the bump.
+# The .bak suffix is needed for macOS compatibility with sed -i.
+pre_bump_hooks = [
+    "sed -i.bak 's/^version = \"[^\"]*\"/version = \"{{version}}\"/' Cargo.toml && rm -f Cargo.toml.bak",
+]
+
+[changelog]
+path = "CHANGELOG.md"
+template = "remote"
+remote = "github.com"
+repository = "dredge"
+owner = "anthonyoteri"
+authors = [
+    { username = "anthonyoteri", signature = "Anthony Oteri" },
+]
+
+[commit_types]
+feat     = { changelog_title = "Features" }
+fix      = { changelog_title = "Bug Fixes" }
+perf     = { changelog_title = "Performance" }
+refactor = { changelog_title = "Refactoring" }
+docs     = { changelog_title = "Documentation" }
+test     = { changelog_title = "Tests" }
+chore    = { changelog_title = "Miscellaneous" }
+ci       = { changelog_title = "CI", omit_from_changelog = true }
+style    = { changelog_title = "Style", omit_from_changelog = true }