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/tests/test_loop_milestones.py

@@ -14,7 +14,14 @@ from nisse_dev.cancel import Cancel
 from nisse_dev.canonical import ToolCall
 from nisse_dev.execution import SSHHost
 from nisse_dev.git import GitRepo
-from nisse_dev.gitea import Comment, GiteaError, Issue, PullRequest, ReviewNote
+from nisse_dev.gitea import (
+    Approval,
+    Comment,
+    GiteaError,
+    Issue,
+    PullRequest,
+    ReviewNote,
+)
 from nisse_dev.loop.agent import LoopAgent
 from nisse_dev.loop.driver import Driver
 from nisse_dev.loop.items import ANSWER, MERGE, WAITING, loop_of
@@ -62,6 +69,10 @@ class Forge:
         self.deps: dict[int, list[int]] = {}  # issue -> the issues it depends on
         self.assigned: list[int] = []  # issues the agent assigned to itself
         self.reassigned: dict[int, str] = {}  # issue -> when assigned again
+        # The trust dial (M3.3): reviews, check statuses, merges by the agent.
+        self.approved: dict[int, list[Approval]] = {}
+        self.status: dict[str, str] = {}  # commit -> combined check state
+        self.agent_merged: list[int] = []
 
     # intake
     def assigned_issues(self, repo: str, assignee: str) -> list[Issue]:
@@ -91,7 +102,40 @@ class Forge:
     # PRs
     def _pr(self, n: int) -> PullRequest:
         p = self.prs[n]
-        return PullRequest(n, p["state"], p["head"], f"https://g/pr/{n}", p["merged"])
+        return PullRequest(
+            n,
+            p["state"],
+            p["head"],
+            f"https://g/pr/{n}",
+            p["merged"],
+            head_sha=self.head_sha(n),
+        )
+
+    def head_sha(self, n: int) -> str:
+        ref = f"refs/heads/{self.prs[n]['head']}"
+        found = subprocess.run(
+            ["git", "--git-dir", str(self.bare), "rev-parse", "--verify", "-q", ref],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        return found.stdout.strip()
+
+    def approve(self, n: int, by: str = "don", sha: str = "") -> None:
+        """A review approving the PR as it is now (or at ``sha``)."""
+        self.approved.setdefault(n, []).append(Approval(by, sha or self.head_sha(n)))
+
+    def approvals(self, repo: str, index: int) -> list[Approval]:
+        return self.approved.get(index, [])
+
+    def checks(self, repo: str, sha: str) -> str:
+        return self.status.get(sha, "none")
+
+    def branch_with_file(self, repo: str, *, base, branch, path, text, message) -> None:
+        git("--git-dir", str(self.bare), "update-ref", f"refs/heads/{branch}", base)
+
+    def delete_branch(self, repo: str, name: str) -> None:
+        git("--git-dir", str(self.bare), "update-ref", "-d", f"refs/heads/{name}")
 
     def find_pull_request(self, repo: str, head: str) -> PullRequest | None:
         for n, p in self.prs.items():
@@ -150,8 +194,15 @@ class Forge:
         return {"protected": self.protected, "user_can_push": not self.protected}
 
     def merge_pull_request(self, repo: str, index: int) -> None:
-        if self.protected:
+        """As protection would have it: an operator's approval of the
+        PR's current commit, or no merge."""
+        head = self.head_sha(index)
+        if self.protected and not any(
+            a.author == "don" and a.commit == head and not a.stale
+            for a in self.approved.get(index, [])
+        ):
             raise GiteaError("gitea POST merge returned 405: approvals needed")
+        self.agent_merged.append(index)
         self.merge(index)
 
     # what the operator does in Gitea