ci(skill-check): scan the docs a contributor reads

The check asserts that every make target named in a doc exists, and its
scanned set was .pi/ plus CLAUDE.md.  Since #50, CONTRIBUTING.md is the
document a *human* goes to for a build command, and it names 21 targets
that nothing verified; README.md names none today and is in for the same
reason.  The script's own header sentence is the argument — a renamed
target sends a person off the same cliff it sends an agent off.

The file list is now one `docs` variable used twice, because the failure
message carried a second copy of it and a second list is a second thing
to forget.  The `[ -d .pi ]` guard went with it: gating the whole run on
.pi/ would make the human-facing half conditional on the agent-facing
one, and an empty list is the same "nothing to scan" exit without the
coupling.

The lefthook glob is that scanned set now rather than
{Makefile,.pi/**/*.md} — #220's smaller half, and it did not fire on
CLAUDE.md either, which the script had read for months.

Verified by planting a bad target rather than by reading the diff: both
matched forms in each of the four scanned surfaces, each naming the
right file; the same two plants pass on the pre-change script; unfenced
prose still does not match; and the hook fires on a staged
CONTRIBUTING.md under the new glob where the old one skipped it.  The
count is unchanged at 47 — the set is a union — so coverage is the only
thing that moved.

Closes #220

lefthook.yml

@@ -38,10 +38,14 @@ pre-commit:
       glob: "*.go"
       run: ./scripts/bindings-check.sh
 
-    # .pi/ documents make targets; a stale one sends an agent off a
-    # cliff with total confidence.  Instant.
+    # The docs document make targets; a stale one sends an agent — or a
+    # contributor reading CONTRIBUTING.md — off a cliff with total
+    # confidence.  The glob is the script's own scanned set, because a
+    # hook that does not fire on a file the check reads is the drift the
+    # check exists to prevent: it was `{Makefile,.pi/**/*.md}` while the
+    # script already read CLAUDE.md.  Instant.
     skill-check:
-      glob: "{Makefile,.pi/**/*.md}"
+      glob: "{Makefile,.pi/**/*.md,AGENTS.md,CLAUDE.md,README.md,CONTRIBUTING.md}"
       run: ./scripts/skill-check.sh
 
     frontend-typecheck: