refactor: migrate to function-based design with multiple improvements

[phsmith]

Summary

• Migrated build system to UV — replaced setup.py, requirements.txt, tox.ini, and MANIFEST.in with pyproject.toml + uv.lock; added ruff and bandit as dev dependencies for linting and security scanning
• Refactored CLI from class-based to function-based design — replaced CircleCIEnvsManage class with a _Ctx dataclass + module-level functions; fixed CIRCLE_TOKEN env var not being picked up when installed as a console script via context_settings={"auto_envvar_prefix": "CIRCLE"}
• Multiple UX improvements — sorted variable listings with count header, f-string logging throughout, no-op validation when no action flags are passed, graceful 404 handling, support for 2-part slugs (vcs_type/org) for context-only operations
• Overhauled CI/CD pipelines — merged publish.yml + release.yml into a single 4-job release pipeline (prepare → publish_pypi ∥ publish_docker → commit_and_release); version bump committed to main only after all publish steps succeed with [skip ci]; added pr-checks.yml with linting, security scan, multi-version build matrix, and Docker build check; added update-changelog.yml triggered on release publish

Test plan

• uv run circleci-env-cli --help works and CIRCLE_TOKEN is picked up from env
• uv run circleci-env-cli -p github/org --list-envs returns a sorted list with count header
• uv run circleci-env-cli -p github/org -c my-context --list-envs works without a project name
• uv run circleci-env-cli -p github/org exits with "nothing to do" error
• uv run ruff check circleci_env_cli.py passes
• uv run bandit -c pyproject.toml circleci_env_cli.py passes
• docker build . succeeds