Eliminate double towncrier run in release workflow (#199)

## Summary

- Added a new `build_quartz` Dagger function that builds the Quartz site from a pre-processed source tree (no towncrier)
- Reordered the release workflow so towncrier runs **once** on the runner, then passes the updated working tree to `build-quartz`
- `build_docs` and `build_changelog` are preserved for standalone use — `build_docs` now delegates to `build_quartz` internally

## Motivation

Previously towncrier ran twice per release: once inside a Dagger container (via `build_docs` → `build_changelog`) and once on the runner to capture CHANGELOG.md changes for the git commit. This was wasteful and fragile — if towncrier behavior changed, the two runs could produce different results.

## Test plan

- [ ] Review diff to confirm workflow step ordering is correct
- [ ] Trigger a release and confirm towncrier runs only once
- [ ] Verify the docs tarball contains the updated CHANGELOG.md
- [ ] `dagger call build-quartz --src=. --version=vX.Y.Z` should work standalone

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/199

docs/reference/tools/dagger.md

@@ -27,8 +27,7 @@ Build engine for BlumeOps CI/CD pipelines. Replaces shell-based build scripts wi
 |----------|-----------|-------------|
 | `build` | `(src, container_name) → Container` | Build a container from `containers/<name>/Dockerfile` |
 | `publish` | `(src, container_name, version, registry?) → str` | Build and push to registry (default: `registry.ops.eblu.me`) |
-| `build_changelog` | `(src, version) → Directory` | Run towncrier to collect changelog fragments |
-| `build_docs` | `(src, version) → File` | Build changelog then Quartz site, return docs tarball |
+| `build_docs` | `(src, version) → File` | Build Quartz docs site, return docs tarball |
 
 ## CLI Examples