docs(examples): add builder-adversary-stateless — context-lean variant

Same pattern + AI-as-adversary verification as builder-adversary-min, but the
role prompts add CONTEXT HYGIENE: /compact at every checkpoint (lossless — state
is on disk), read diffs not trees, spill bulk output to files, adversary loads
only {plan, STATUS, diff}. Loop agents non-resumed → fresh session per phase.
Targets cache-read (the dominant cost in a long loop) without changing what the
agents do or how they verify.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/builder-adversary-stateless/machine-docs/.gitkeep

@@ -0,0 +1,2 @@
+# Coordination / loop-state files live here at runtime (phase-namespaced STATUS / REVIEW / BACKLOG /
+# JOURNAL, plus the ADVERSARY-INBOX.md / BUILDER-INBOX.md side-channels). The loop pair populates it.