chore: docs/ prose-only gate + drop validate.py legacy fallback

[rafael5]

Summary

Two small cleanups bundled — the cross-repo docs/ guardrail (option #1 from the post-Phase-0 audit) and the legacy fallback removal (option #3).

1. make check-docs-prose

Cross-repo guardrail. docs/ holds only human-readable prose (.md, .markdown, images, .gitkeep); non-prose artifacts belong under dist/, examples/, templates/, or a top-level domain dir. Engine-free find-based check, wired into CI alongside the existing manifest drift gate.

Same target ships in all four m-dev-tools repos so future contributors / agents get a consistent fail-fast signal — preventive, not corrective.

2. Drop validate.py legacy docs/integrated/ fallback

The relocation back to top-level integrated/ (PR #6) is committed and Phase 0 smoke is green against the new layout. The fallback was a migration-window affordance; with the window closed, keeping it perpetuates the idea that docs/integrated/ is ever acceptable.

 def _integrated_dir(project_root: Path) -> Path:
-    """Resolve the integrated/ directory location.
-
-    The canonical layout has `integrated/` at the repo root, ...
-    fallback is kept for one release cycle so a stale checkout still
-    validates.
-    """
-    canonical = project_root / "integrated"
-    if canonical.is_dir():
-        return canonical
-    legacy = project_root / "docs" / "integrated"
-    if legacy.is_dir():
-        return legacy
-    return canonical
+    """Resolve the integrated/ directory location.
+
+    `integrated/` lives at the repo root, alongside `per-source/`,
+    `schemas/`, `sources/`, and `src/`.
+    """
+    return project_root / "integrated"

Test plan

• make check-docs-prose green locally
• make check-manifest green locally
• make test — 164 passed
• CI green on this PR