M3.3: the trust dial (the agent merges approved PRs)

A project's trust.merge is operator (the default) or approved (L3, D5,
#65). Set to approved, the harness, never the model, merges a PR in the
PR watch once an approving review from a gitea.operators account is on
its current commit, its checks pass (none at all counts; pending
waits) and it's mergeable; the item then goes on as after any merge.
Each reason to hold off (failed checks, conflicts, a refused merge) is
said once per commit.

Gitea enforces it: protection requires approvals from an operators-only
whitelist and dismisses stale ones. At start, for each approved
project, the agent checks by behaviour that its own unapproved merge is
refused, with a throwaway branch and PR made through the API (never the
workspace); if it isn't refused, the operator merges until a restart.

- gitea: approvals, combined check status, a PR's head commit and
  mergeability, API-made branches and their deletion.
- Test and bring-up repositories get the protection.
- Tests: the decision (unit), the hard wall on a real Gitea
  (test_trust_live), an e2e test where the operator only approves, and
  a walk property, merges_only_approved, checked against a mutant that
  ignores which commit was approved.

Closes #65

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

tests/integration/test_trust_live.py

@@ -0,0 +1,75 @@
+"""The trust dial's hard wall, on a real Gitea (D4, D5; M3.3): with the
+protection a `merge: approved` project has, the agent's own token can't
+merge its PR until an operator approves the PR as it is now; it can then;
+and a commit pushed after the approval needs approving again."""
+
+import base64
+import uuid
+from pathlib import Path
+
+import pytest
+from e2e_support import fresh_repo, operator_gitea
+
+from nisse_dev.config import AgentConfig
+from nisse_dev.gitea import GiteaClient, GiteaError
+
+pytestmark = pytest.mark.integration
+
+
+def add_file(gitea: GiteaClient, repo: str, branch: str, path: str) -> None:
+    gitea._request(
+        "POST",
+        f"/repos/{repo}/contents/{path}",
+        json={
+            "content": base64.b64encode(f"{path}\n".encode()).decode(),
+            "message": f"add {path}",
+            "branch": branch,
+        },
+    )
+
+
+def approve(operator: GiteaClient, repo: str, pr: int) -> None:
+    operator._request(
+        "POST", f"/repos/{repo}/pulls/{pr}/reviews", json={"event": "APPROVED"}
+    )
+
+
+def test_merging_needs_the_operators_approval_of_the_pr_as_it_is(
+    tmp_path: Path, live_config: AgentConfig, gitea: GiteaClient
+) -> None:
+    operator = operator_gitea(live_config.gitea_url)
+    run = uuid.uuid4().hex[:8]
+    with fresh_repo(operator, run, tmp_path / "keys", agent=gitea.whoami()) as repo:
+        name = repo.name
+        # The agent's PR, its branch made through the API.
+        gitea.branch_with_file(
+            name, base="main", branch="m-1", path="a.txt", text="a\n", message="a"
+        )
+        pr = gitea.open_pull_request(name, head="m-1", base="main", title="a", body="")
+        with pytest.raises(GiteaError):
+            gitea.merge_pull_request(name, pr.number)  # no approval yet
+
+        approve(operator, name, pr.number)
+        [approval] = gitea.approvals(name, pr.number)
+        head = gitea.pull_request(name, pr.number).head_sha
+        assert (approval.author, approval.commit) == (operator.whoami(), head)
+        gitea.merge_pull_request(name, pr.number)  # approved: the agent may
+        assert gitea.pull_request(name, pr.number).merged
+
+        # Approved, then changed: the approval no longer stands.
+        gitea.branch_with_file(
+            name, base="main", branch="m-2", path="b.txt", text="b\n", message="b"
+        )
+        second = gitea.open_pull_request(
+            name, head="m-2", base="main", title="b", body=""
+        )
+        approve(operator, name, second.number)
+        add_file(gitea, name, "m-2", "c.txt")  # a fix pushed after the approval
+        after = gitea.pull_request(name, second.number).head_sha
+        assert not [
+            a
+            for a in gitea.approvals(name, second.number)
+            if a.commit == after and not a.stale
+        ]
+        with pytest.raises(GiteaError):
+            gitea.merge_pull_request(name, second.number)