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

docs/Architecture.md

@@ -726,9 +726,12 @@ Installing on any other host is dangerous.
 - **The PR may have moved on meanwhile.** After pushing a fix, the agent checks the
   PR: if it merged before the fix landed, the fix goes out as a new PR; if it was
   closed, the agent says so and asks.
-- **Merging follows the trust dial** (L3). Today the operator merges. If the dial
-  lets the agent merge its own PRs once checks pass, branch protection must require
-  an approval the agent's account can't give, so Gitea enforces the rule.
+- **Merging follows the trust dial** (L3), set per project (`trust.merge`). By
+  default the operator merges. Set to `approved`, the agent merges a PR once an
+  operator has approved its current commit and its checks pass; branch
+  protection requires that approval from an operators-only whitelist and
+  dismisses stale approvals, so Gitea enforces the rule, and the agent checks at
+  start that its own unapproved merge is refused.
 
 ### D6. Action classes