Move non-core root reference docs under docs/reference

docs/reference/ANTI_DRIFT.md

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+# Anti-Drift Policy
+
+The grind repo must reflect real implementation.
+
+Do NOT:
+
+- Document future features as complete
+- Leave outdated architecture notes
+- Allow staging/symlink model references to remain
+- Duplicate filesystem rules outside agent
+
+When architecture changes:
+
+1. Update README
+2. Update CONSTRAINTS
+3. Update relevant architecture doc
+4. Update SESSION_LOG
+5. Close thread in OPEN_THREADS if resolved