fix(ci): make every manual gate default to a value that matches nothing

A manual pipeline creation instantiates every file in .crow/, and a declared
variable default is applied even when the run never passed that variable. So a
gate like target_arch, defaulting to amd64, matched its own amd64 matrix rows on
any manual run of any pipeline. Triggering a weekly-audit-missing run for one
slot started build-all-versions (including its Upload package indexes step) and
process-updates across every row.

#155 fixed the three pipelines that had no gate at all; these six had a gate
whose default was permissive, which left them just as exposed.

- add a 'none' option to each gate variable and default to it, so a manual run
  must name its target explicitly
- record why the default must match nothing, next to the default itself

Cron triggers are unaffected: they match on the cron name, not the variable.

.crow/weekly-audit-missing.yaml

@@ -7,9 +7,13 @@
 #             ("all" = every os/arch).
 # Arch placement is via the group label (rpkgs-amd64, rpkgs-arm64).
 variables:
+  # Gates this pipeline. A manual pipeline creation instantiates every file in
+  # .crow/, and a declared default is applied even when the run never passed
+  # this variable, so the default must be a value that matches no matrix row.
   weekly_audit_missing:
-    description: "Manual run target: a specific <os>-<arch>, or 'all' for every os/arch."
+    description: "Manual run target: a specific <os>-<arch>, 'all' for every os/arch, or 'none' to run nothing."
     options:
+      - none
       - all
       - alpine-322-amd64
       - alpine-322-arm64
@@ -29,7 +33,7 @@ variables:
       - ubuntu-2404-arm64
       - ubuntu-2604-amd64
       - ubuntu-2604-arm64
-    default: all
+    default: none
 
 when:
   - event: cron