fix(tooling): run the docs-drift workflow when the mapper itself changes

CI on the first push of this PR showed the gap: `Flag docs affected by code changes`
never ran, because the workflow triggers on `paths: ['packages/**']` and this PR
touches only `scripts/` and `.github/`. So the self-test step added one commit ago was
added but never exercised, and editing `affected-docs.mjs` was the one change its own
guard could structurally never see — the tool is unguarded exactly when it is being
modified.

Adds the mapper's own paths to the trigger. A PR touching only these gets the benign
"No hand-written docs reference the 0 changed package(s). ✅" branch the comment step
already has, which is the correct answer, and the self-test now runs on the change most
likely to break it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QoA8AV99Ss1RRkLLAYmDzq

Author: claude

.github/workflows/docs-drift-check.yml

@@ -11,6 +11,13 @@ on:
     types: [opened, synchronize, reopened]
     paths:
       - 'packages/**'
+      # The mapper and this workflow, so a change to the guard runs the guard. Without
+      # these, editing `affected-docs.mjs` was the one change its own self-test could
+      # never see — a `packages/**`-only trigger means the tool is unguarded exactly
+      # when it is being modified. A PR that touches only these gets the benign
+      # "0 changed package(s) ✅" comment, which is the correct answer.
+      - 'scripts/docs-audit/**'
+      - '.github/workflows/docs-drift-check.yml'
 
 permissions:
   contents: read