Add automated release workflow with changelog integration

[Copilot]

Implements manual workflow for creating releases that automatically updates the changelog from unreleased entries, creates tags, and publishes releases with extracted release notes.

Changes

Workflow (.github/workflows/release.yml)

• Manual trigger via workflow_dispatch with version input (semantic versioning)
• Validates version format, checks for duplicate tags and empty releases
• Commits changelog update to main, creates annotated tag, publishes GitHub release

Scripts

• bin/prepare-release.sh: Moves [Unreleased] section content to new version header with current date using awk
• bin/extract-release-notes.sh: Extracts changelog content for specific version to populate release notes

Example usage:

# Actions → Create Release → Run workflow
# Input: 6.7.0
# Result: Tag v6.7.0 created, CHANGELOG.md updated:
## [Unreleased]

## [6.7.0] - 2026-02-07
### Fixed
- UriRetriever: Include actual media type in InvalidSchemaMediaTypeException message (#872)

All scripts include validation and error handling for edge cases (empty unreleased section, duplicate versions, missing changelog).

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