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

CLAUDE.md

@@ -161,7 +161,7 @@ make ui-test          # Vitest component/store suite in a real browser (no app)
 make ui-visual        # Same, including toMatchScreenshot comparisons
 make ui-setup         # Install the Vitest provider's own Chromium (once)
 make bindings-check   # Fail if frontend/bindings is stale vs the Go bindings
-make skill-check      # Fail if .pi/ documents a make target that doesn't exist
+make skill-check      # Fail if a doc names a make target that doesn't exist
 make commit-check     # Fail if a commit subject is not a Conventional Commit
 make lint             # golangci-lint v2 (strict), all three build configurations
 make test             # All tests with race detector, all three build configurations