ci: add changelog metadata tracking for automated releases

What does this MR do and why?

This MR adds metadata tracking to CHANGELOG.md to record the commit SHA and timestamp used during changelog generation. This enables better traceability and helps verify which changes are included in each release.

Related to #183

Implementation

• Created scripts/changelog-metadata.sh - a unified script that handles both updating and extracting changelog metadata
• Updated .gitlab-ci.yml to call the script during the release_create_mr job
• Updated cliff.toml to include a metadata placeholder in the footer template
• Added documentation in doc/process.md explaining the metadata feature

Metadata Format

The metadata is stored at the end of CHANGELOG.md as a single-line JSON comment:

<!-- changelog_metadata: {"generated_at":1765294058,"generated_at_iso":"2025-12-09T15:27:38Z","commit_sha":"abc123def456789"} -->

Fields:

• commit_sha: Full commit SHA from which the changelog was generated
• generated_at: Unix timestamp when the changelog was generated
• generated_at_iso: ISO 8601 formatted timestamp (UTC)

Usage

# Extract all metadata (pretty-printed JSON)
./scripts/changelog-metadata.sh get

# Extract specific field
./scripts/changelog-metadata.sh get CHANGELOG.md commit_sha

# Update metadata (CI usage)
./scripts/changelog-metadata.sh update CHANGELOG.md "$CI_COMMIT_SHA"

Testing

• Script passes shellcheck validation
• Comprehensive error handling tested
• Metadata will be automatically populated on the next automated weekly release