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/repair-built-stamp.yaml

@@ -19,12 +19,16 @@
 #     --var repair_built_stamp=arm64 --var OS=alpine --var OS_VERSION=3.22 \
 #     --var R_VERSION=4.5.3 --var dry_run=true devxy/build-cran-binaries
 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.
   repair_built_stamp:
-    description: 'Architecture of the slot to repair. Also gates this pipeline.'
+    description: 'Architecture of the slot to repair, or "none" to run nothing.'
     options:
+      - none
       - amd64
       - arm64
-    default: arm64
+    default: none
   OS:
     description: 'Base OS image name.'
     options: