Add towncrier changelog system (#86)

## Summary
- Configure towncrier with custom types (feature, bugfix, infra, doc, misc)
- Build initial v0.1.0 changelog from zk management log entries
- Integrate towncrier into build-blumeops workflow
- Update README to mark Phase 1b complete

## How It Works
1. Add changelog fragments to `docs/changelog.d/` as `<id>.<type>.md`
2. When running build-blumeops workflow, towncrier collects fragments
3. CHANGELOG.md is updated and fragments are removed
4. Changes are committed back to main before docs build

## Testing
- [x] Tested `uvx towncrier build` locally
- [ ] Test workflow execution (after merge)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/86

towncrier.toml

@@ -0,0 +1,35 @@
+# Towncrier configuration for BlumeOps changelog
+# https://towncrier.readthedocs.io/
+
+[tool.towncrier]
+directory = "docs/changelog.d"
+filename = "docs/CHANGELOG.md"
+package = ""
+title_format = "## [{version}] - {project_date}"
+issue_format = ""
+underlines = ["", "", ""]
+
+[[tool.towncrier.type]]
+directory = "feature"
+name = "Features"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "bugfix"
+name = "Bug Fixes"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "infra"
+name = "Infrastructure"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "doc"
+name = "Documentation"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "misc"
+name = "Miscellaneous"
+showcontent = false