Replace towncrier with changie for changelog management

[Boomatang]

Closes #65

Summary

Migrates changelog management from towncrier (Python) to changie (Go), integrating the changie binary as a lazy Zig build dependency so contributors no longer need Python or pip installed.

• Changie binary is fetched on demand via build.zig.zon lazy dependencies for linux/darwin on amd64/arm64
• New build steps: changie:add, changie:batch, changie:merge, changie:version
• CI changelog fragment check simplified from a towncrier semantic check to a git-diff-based check for new .changes/unreleased/*.yaml files
• CHANGELOG.md heading hierarchy corrected to use h1 for the document title, h2 for versions, h3 for categories
• Historical changelogs preserved as individual .changes/X.X.X.md files for changie to manage
• README updated with the new changie-based contributor and release workflows

Addressing Issue #65

The issue requested switching from towncrier to changie, noting that the Zig build system makes it easier to manage development tools distributed as binaries. This PR fulfills that by:

• Changie replaces towncrier entirely: the towncrier.toml config and changelog.d/ directory are removed; .changie.yaml and .changes/ take their place
• Binary managed via Zig: changie is pulled as a lazy dependency, avoiding any external toolchain requirement beyond Zig itself
• Workflows updated: both CI and the documented release process reflect the new tool

The issue had no explicit definition of done or acceptance criteria.