Upgrade Dagger from v0.19.11 to v0.20.0

Pin dagger CLI to 0.20.0 in mise.toml (was "latest"), bump engine
version in dagger.json. New .dagger/.gitattributes and .gitignore
generated by v0.20.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dagger/.gitattributes

@@ -0,0 +1 @@
+/sdk/** linguist-generated

.dagger/.gitignore

@@ -0,0 +1,4 @@
+/.venv
+/**/__pycache__
+/sdk
+/.env

dagger.json

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

mise.toml

@@ -1,3 +1,3 @@
 [tools]
 prek = "latest"
-dagger = "latest"
+dagger = "0.20.0"