Merge pull request 'Record default-branch protection as a B2 requirement' (#19) from b2-branch-protection into main

docs/Architecture-Build-Spec.md

@@ -550,6 +550,25 @@ PR_OPEN
   - *Outbound:* `git` + Gitea API to push branches and open PRs, via a dedicated
     Gitea **service account** with a scoped token. Record any runnable test service's
     URL/port in the PR description.
+  - **The project repo's default branch must be protected, and this is what makes
+    the PR gate real.** The service account needs write access to push branches —
+    and in Gitea, *write access includes merging pull requests*. Repository
+    ownership does not help: an account that merely collaborates can still merge,
+    and one that owns the repo can merge and delete it. So without branch
+    protection the agent can merge its own PR, and the review gate holds only
+    because the agent's code does not call the merge endpoint. That is precisely
+    the advisory guarantee A4 exists to rule out.
+    - Required on the default branch: **block direct pushes** by the agent
+      account, and **restrict merging** to the operator (a merge whitelist, or
+      required approvals that the agent's own account cannot satisfy).
+    - Feature-branch pushes are unaffected, so the agent's normal workflow is
+      unchanged.
+    - **Verify it rather than assume it.** The property is observable from
+      outside: the agent's own token must be refused when it attempts to merge
+      its own PR and when it pushes to the default branch. Assert both, so that
+      relaxing the protection later fails a test instead of silently removing
+      the gate. (Discovered the hard way — before protection was configured, the
+      agent's token merged its own PR successfully.)
   - *Inbound:* react to PR comments, commits, pushes. **Poll** the Gitea API
     (15–30s) rather than take webhooks, to keep the VM outbound-only. Intake is
     structured JSON — never via a chat bridge.