ci: add workflow_dispatch to force a full --all-files prek run

Lets us validate the prek spike against the whole tree (mypy included)
on demand instead of waiting for the nightly cron. Spike-only.

.github/workflows/pre-commit.yml

@@ -12,6 +12,11 @@ on:
   # importing test) can pass every PR yet leave master red. This catches that.
   schedule:
     - cron: "0 6 * * *"
+  # Spike-only: lets us force a full --all-files run on demand (workflow_dispatch
+  # has no diffable base, so "Determine changed files" below falls back to
+  # mode=all) to validate prek against the whole tree without waiting for the
+  # nightly cron. Remove if/when this spike is promoted or dropped.
+  workflow_dispatch:
 
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