feat: support and test MDCPS reviewer identity per #107

- Update v2 test config: mdcps reviewer username '913443' (distinct from author 'jcwalker3'), author 'jcwalker3'
- Adjust TBD negative test to use mutate (post-provisioning)
- Add mdcps-reviewer example to gitea-mcp.example.json and README
- Verifies distinct identities, reviewer cannot create/push, author cannot review/merge

Closes #107

Checks:
- config tests pass
- no secrets in changes (usernames only)
- py_compile and diff clean

gitea-mcp.example.json

@@ -21,6 +21,18 @@
       "default_owner": "Contractor",
       "execution_profile": "mdcps"
     },
+    "mdcps-reviewer": {
+      "base_url": "https://gitea.dadeschools.net",
+      "username": "913443",
+      "auth": {
+        "type": "keychain",
+        "id": "mdcps.gitea.reviewer.token"
+      },
+      "default_owner": "MDCPS",
+      "execution_profile": "mdcps-reviewer",
+      "allowed_operations": ["read", "review", "approve", "merge"],
+      "forbidden_operations": ["branch.push", "pr.create"]
+    },
     "prgs-env": {
       "base_url": "https://gitea.prgs.cc",
       "username": "jcwalker3",