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

@@ -3,12 +3,16 @@
 # Variables are declared so the manual-run form exposes them (crow #1165);
 # they are merged with build-all-versions' identical declarations.
 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.
   target_arch:
-    description: 'Architecture to build.'
+    description: 'Architecture to build, or "none" to run nothing.'
     options:
+      - none
       - amd64
       - arm64
-    default: amd64
+    default: none
   OS:
     description: 'Base OS image name.'
     options: