Upgrade Dagger from v0.19.11 to v0.20.0 (#285)

## Summary
- Bump Dagger engine version from v0.19.11 to v0.20.0 in `dagger.json`
- Pin dagger CLI to `0.20.0` in `mise.toml` (was `"latest"`)
- Regenerated `.dagger/uv.lock` (new SDK deps: httpcore, beartype bump)

## Testing
- [x] `dagger call validate-workflows --src=.` passes on v0.20.0
- [ ] CI build workflow passes

Reviewed-on: #285

dagger.json

@@ -1,6 +1,6 @@
 {
   "name": "blumeops-ci",
-  "engineVersion": "v0.19.11",
+  "engineVersion": "v0.20.0",
   "sdk": {
     "source": "python"
   },