feat(ci): make OS and OS_VERSION manual-run dropdowns

Give the OS and OS_VERSION form variables explicit option lists like
target_arch and R_VERSION already have, so the manual-run form presents
dropdowns instead of free-text. Crow form variables are independent, so the
operator must still pick a coherent OS + OS_VERSION combination.

.crow/build-all-versions-install-deps.yaml

@@ -10,10 +10,21 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
+    description: "Base OS image name."
+    options:
+      - alpine
+      - redhat
+      - ubuntu
     default: alpine
   OS_VERSION:
-    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
+    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
+    options:
+      - "3.24"
+      - "8"
+      - "9"
+      - "10"
+      - "jammy"
+      - "noble"
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."

.crow/build-all-versions.yaml

@@ -11,10 +11,21 @@ variables:
       - arm64
     default: amd64
   OS:
-    description: "Base OS image name (e.g. alpine, redhat, ubuntu)."
+    description: "Base OS image name."
+    options:
+      - alpine
+      - redhat
+      - ubuntu
     default: alpine
   OS_VERSION:
-    description: "OS version / image tag (e.g. 3.24, 9, jammy, noble)."
+    description: "OS image tag. Must match OS (alpine: 3.24; redhat: 8/9/10; ubuntu: jammy/noble)."
+    options:
+      - "3.24"
+      - "8"
+      - "9"
+      - "10"
+      - "jammy"
+      - "noble"
     default: "3.24"
   R_VERSION:
     description: "Primary R version under /opt/R."