ci: Add manual dry-run mode to release workflow

[cdecker]

Summary

• Added workflow_dispatch trigger to the release workflow, allowing manual testing without publishing
• Modified tag parsing to support both tag-triggered releases and manual dry-run mode
• Added conditionals to skip cargo publish and GitHub release creation during dry-runs
• Added a summary step at the end of dry-runs showing what checks passed and how to publish for real

Changes

The workflow now supports two modes:

Tag-triggered mode (existing behavior):

• Push a tag like mycrate-v1.2.3
• All validation checks run
• Publishes to crates.io and creates GitHub release

Manual dry-run mode (new):

• Navigate to Actions → Publish Crate → Run workflow
• Enter crate name and version
• All validation checks run (version match, changelog format, changelog entry, semver, tests)
• Skips cargo publish and GitHub release creation
• Shows summary with instructions to create the actual tag

Test plan

• Verify workflow file syntax is valid
• Test manual dry-run trigger from GitHub Actions UI
• Verify dry-run skips publish steps
• Verify tag-triggered mode still works as before

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