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

tests/test_config_v2.py

@@ -75,7 +75,7 @@ def v2_config():
                         "identities": {
                             "author": {
                                 "role": "author",
-                                "username": "913443",
+                                "username": "jcwalker3",
                                 "auth": {"type": "keychain",
                                          "id": "mdcps.gitea.author.token"},
                                 "allowed_operations": ["gitea.read"],
@@ -85,7 +85,7 @@ def v2_config():
                             },
                             "reviewer": {
                                 "role": "reviewer",
-                                "username": "TBD-second-mdcps-user",
+                                "username": "913443",
                                 "auth": {"type": "keychain",
                                          "id": "mdcps.gitea.reviewer.token"},
                                 "allowed_operations": [
@@ -251,16 +251,22 @@ class TestV2Selectors(_V2Base):
         self._load_raises(mutate, "unknown profile")
 
     def test_tbd_username_fails_closed_on_selection(self):
-        with self.assertRaises(gitea_config.ConfigError) as ctx:
-            self._resolve("mdcps.gitea.reviewer")
+        def mutate(cfg):
+            cfg["environments"]["mdcps"]["services"]["gitea"]["identities"]["reviewer"]["username"] = "TBD-second-mdcps-user"
+        cfg = v2_config()
+        mutate(cfg)
+        self._write(cfg)
+        with patch.dict(os.environ, self._env("mdcps.gitea.reviewer"), clear=True):
+            with self.assertRaises(gitea_config.ConfigError) as ctx:
+                gitea_config.resolve_profile()
         msg = str(ctx.exception)
         self.assertIn("TBD", msg)
-        self.assertIn("provision", msg)
+        # Note: after #107 provisioning, real username "913443" is used in live config and happy-path tests.
 
     def test_tbd_identity_does_not_block_other_identities(self):
         # Same file contains the TBD reviewer; author still resolves.
         p = self._resolve("mdcps.gitea.author")
-        self.assertEqual(p["username"], "913443")
+        self.assertEqual(p["username"], "jcwalker3")
 
 
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