chore: remove the GitHub Actions workflow — there is no CI

The repo lives on a Gitea server and CI/CD is a long way off, so
.github/workflows/content-build.yml never ran. A workflow nobody executes is
worse than none: it read as a safety net that wasn't there.

Removing it moves the `--check` gate somewhere real. It is now enforced by
tools/content_build/tests/test_real_content.py, which runs check() against the
live bible under pytest — so the gate is a test you actually run, not a job
that never fired.

The world-building skill's verify step now says plainly that nothing runs these
for you, and adds the client suite to the checklist: content/world/ is a CLIENT
INPUT (content_db.gd loads it, test_content_db.gd asserts on it), so a content
change can break the client. That is exactly what the Duncarrow purge did, and
with no CI there is nothing else to catch it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/content_build/tests/test_real_content.py

@@ -1,6 +1,8 @@
-"""Smoke gate: the real Margreave bible builds clean. Local mirror of the CI
-content-build workflow's `--check` step — asserts freshness/validity only,
-never specific entities (that's what made the old Duncarrow-era test brittle)."""
+"""Smoke gate: the real Margreave bible builds clean.
+
+There is no CI — this test IS the `--check` gate (freshness, validity, no
+orphans, no secrecy leaks). It asserts those properties only, never specific
+entities; pinning entities is what made the old Duncarrow-era test brittle."""
 
 from pathlib import Path