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

nisse-dev/src/nisse_dev/gitea.py

@@ -10,6 +10,7 @@ idempotent across a crash: boot reconciliation (L8) asks Gitea whether the
 effect already happened rather than repeating it.
 """
 
+import base64
 from dataclasses import dataclass
 from typing import Any
 
@@ -33,6 +34,18 @@ class PullRequest:
     head: str
     url: str
     merged: bool = False
+    head_sha: str = ""  # the commit the PR is at now
+    mergeable: bool | None = None  # None: Gitea didn't say
+
+
+@dataclass(frozen=True)
+class Approval:
+    """An approving review, and the commit it approved (L3: approvals bind
+    to exactly what was approved)."""
+
+    author: str
+    commit: str
+    stale: bool = False  # Gitea's mark: commits were pushed since
 
 
 @dataclass(frozen=True)
@@ -286,6 +299,47 @@ class GiteaClient:
                 break
         return when
 
+    def approvals(self, repo: str, index: int) -> list[Approval]:
+        """The PR's approving reviews that still stand: not dismissed."""
+        return [
+            Approval(
+                author=(review.get("user") or {}).get("login", ""),
+                commit=str(review.get("commit_id") or ""),
+                stale=bool(review.get("stale")),
+            )
+            for review in (
+                self._request("GET", f"/repos/{repo}/pulls/{index}/reviews") or []
+            )
+            if review.get("state") == "APPROVED" and not review.get("dismissed")
+        ]
+
+    def checks(self, repo: str, sha: str) -> str:
+        """A commit's combined check status: success, pending, failure or
+        error; "none" when no check reported on it at all."""
+        data = self._request("GET", f"/repos/{repo}/commits/{sha}/status") or {}
+        if not data.get("total_count") and not data.get("statuses"):
+            return "none"
+        return str(data.get("state") or "pending")
+
+    def branch_with_file(
+        self, repo: str, *, base: str, branch: str, path: str, text: str, message: str
+    ) -> None:
+        """A new branch off ``base`` with one file added, made through the
+        API: no clone, no workspace."""
+        self._request(
+            "POST",
+            f"/repos/{repo}/contents/{path}",
+            json={
+                "content": base64.b64encode(text.encode()).decode(),
+                "message": message,
+                "branch": base,
+                "new_branch": branch,
+            },
+        )
+
+    def delete_branch(self, repo: str, name: str) -> None:
+        self._request("DELETE", f"/repos/{repo}/branches/{name}")
+
     def repository(self, repo: str) -> dict[str, Any]:
         """The repository as this account sees it, permissions included."""
         return dict(self._request("GET", f"/repos/{repo}"))
@@ -320,6 +374,8 @@ def _pull_request(data: dict[str, Any]) -> PullRequest:
         head=(data.get("head") or {}).get("ref", ""),
         url=data.get("html_url", ""),
         merged=bool(data.get("merged")),
+        head_sha=(data.get("head") or {}).get("sha", ""),
+        mergeable=data.get("mergeable"),
     )