feat: `semrel changelog` — generate/update CHANGELOG.md without releasing

[mwaldheim]

Summary

A standalone semrel changelog subcommand that generates or updates the project changelog from unreleased commits without performing a full release.

Motivation

There are common workflows where only the changelog needs to be refreshed:

• Previewing what the next release notes would look like in a PR
• Maintaining a CHANGELOG.md that's committed to the repo continuously (not just at release time)
• Generating release notes for a draft PR description

Currently this requires running semrel release --dry-run and manually extracting the changelog output.

Proposed CLI

# Preview to stdout
semrel changelog

# Write/update CHANGELOG.md in place
semrel changelog --write

# Output in a specific format
semrel changelog --format markdown
semrel changelog --format json

# Limit to last N releases
semrel changelog --since v1.0.0

Behaviour

• Uses the same generator plugins already configured in .semrel.yaml
• Respects SEMREL_PLUGIN_TEMPLATE and other generator env-vars
• Does not create tags, push anything or invoke provider/hook plugins
• Exit code 0 when unreleased commits are found, exit 2 when there is nothing to release (same as dry-run)

Acceptance criteria

• semrel changelog command implemented
• --write flag updates the file in-place (prepend to existing CHANGELOG.md)
• Works with all generator plugins (changelog-md, changelog-html, release-notes)
• Usable in CI as a read-only step