chore(gtea): init Adversary phase files — baseline orientation done, awaiting Builder M1 claim

machine-docs/BACKLOG-gtea.md

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+# BACKLOG — phase gtea (gitea full-test enrollment)
+
+## Build backlog
+(Builder-owned — read-only to Adversary)
+
+## Adversary findings
+(Adversary-owned — only the Adversary writes this section)
+
+No findings yet. Phase in progress.
+