Revamp CI: branching flow, updated actions, preview/release publishing

[shankarpandala]

Motivation

• Modernize and extend the CI/CD workflow to support a staged branch promotion model and automated preview/release publishing.
• Update action versions and expand the test matrix to include newer Python versions and non-failing matrix runs.

Description

• Replaced the old .github/workflows/ci.yml with a new workflow that triggers on push, pull_request, and workflow_dispatch for main, dev, qa, and release branches and added permissions: contents: read.
• Expanded the test matrix to include Python 3.13, set fail-fast: false, and upgraded actions to actions/checkout@v4 and actions/setup-python@v5 while updating codecov to v4.
• Added publish-preview job that runs on pushes to qa, sets a preview .dev<run_number> version in pyproject.toml, builds, and uploads to TestPyPI using the TEST_PYPI_API_TOKEN secret.
• Added publish-release job that runs on pushes to release, builds, and publishes to PyPI with PYPI_API_TOKEN, and added a lint job pinned to Python 3.13 running flake8, black --check, and mypy.
• Updated docs/DEVELOPMENT.md to document the branching and promotion flow, CI publishing behavior, and required repository secrets.

Testing

• The workflow defines automated test execution with pytest --cov=pdf_splitter, linting with flake8, black --check, and mypy, and coverage upload via codecov; no CI run results are included in this PR.
• No automated publishing was executed as part of this change, since publish-preview and publish-release are gated by branch and event conditions and require repository secrets to run.

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Codex Task