feat(#2): consume GET /me + GET /capabilities via --whoami one-shot

v1 coverage-audit slice (capabilities+me). Both endpoints had no
caller; add them as cheap boot-time debug primitives.

- sessions.py: get_me (GET /me — identity/whoami) + get_capabilities
  (GET /capabilities — Echo ephemeral-template discovery). Mirror
  get_persona_state: 200 -> parsed dict verbatim, non-200 ->
  SessionApiFailed. Freeform dicts (frozen OpenAPI types both as
  objects).
- cli.py: new --whoami one-shot mode (mirrors --send). Fetches both,
  prints an identity + capabilities report, exits. Standalone probe:
  mutually exclusive with --send/--session/--new/--agent; opens no
  session. New ParsedArgs.whoami field + main() dispatch.
- Contract #2 amended (2 FNs) + validated. TDD: 5 wrapper tests +
  5 cli tests (validation + mode + error). Coverage map: REST 9/40.
  Suite 538 green; touched code ruff-clean.

Audit note: /capabilities is the Echo ephemeral-template discovery
endpoint, not a generic server-caps endpoint (coverage-map framing
corrected). TUI-surfacing of /me + /capabilities deferred.

tests/test_cli.py

@@ -1564,3 +1564,72 @@ class TestBifrostBindCli:
         )
         rc = await _amain(args)
         assert rc == 22
+
+
+class TestWhoami:
+    """--whoami one-shot probe (slice: capabilities+me): GET /me + GET /capabilities."""
+
+    def test_whoami_standalone_accepted(self) -> None:
+        """whoami_standalone_accepted: --whoami alone → valid; whoami=True, no turn flags."""
+        args = _parse_args(["--whoami", "--api-key", "k"])
+        assert args.whoami is True
+        assert args.send_content is None
+        assert args.session_id is None
+        assert args.new is False
+
+    def test_whoami_with_send_rejected(self) -> None:
+        """whoami_with_send_rejected [adversarial]: --whoami + --send → UsageError."""
+        with pytest.raises(UsageError, match="standalone probe"):
+            _parse_args(["--whoami", "--send", "hi", "--api-key", "k"])
+
+    def test_whoami_with_new_rejected(self) -> None:
+        """whoami_with_new_rejected [adversarial]: --whoami + --new → UsageError."""
+        with pytest.raises(UsageError, match="standalone probe"):
+            _parse_args(["--whoami", "--new", "--agent", "m", "--api-key", "k"])
+
+    @respx.mock
+    def test_whoami_mode_prints_report(self, capsys: pytest.CaptureFixture[str]) -> None:
+        """whoami_mode_prints_report [happy,tracer]: /me + /capabilities → stdout report; exit 0."""
+        respx.get("https://w.example/me").mock(
+            return_value=httpx.Response(
+                200,
+                json={
+                    "user_id": "alice",
+                    "scopes": ["conversations.read", "conversations.write"],
+                    "tier": "user",
+                    "key_id": "a1b2c3d4",
+                },
+            )
+        )
+        respx.get("https://w.example/capabilities").mock(
+            return_value=httpx.Response(
+                200,
+                json={
+                    "ephemeral_templates": {
+                        "echo": {
+                            "allowed_models": ["glm5-turbo"],
+                            "default_model": "glm5-turbo",
+                            "system_prompt_max_bytes": 32768,
+                        }
+                    }
+                },
+            )
+        )
+        rc = main(["--whoami", "--api-key", "k", "--server", "https://w.example"])
+        assert rc == 0
+        out = capsys.readouterr().out
+        assert "user_id: alice" in out
+        assert "tier: user" in out
+        assert "key_id: a1b2c3d4" in out
+        assert "ephemeral_template echo" in out
+        assert "glm5-turbo" in out
+
+    @respx.mock
+    def test_whoami_me_auth_failure_exits_20(self, capsys: pytest.CaptureFixture[str]) -> None:
+        """whoami_me_auth_failure [error]: /me 401 → exit 20 [session_api_failed]."""
+        respx.get("https://w.example/me").mock(
+            return_value=httpx.Response(401, json={"detail": "auth_invalid"})
+        )
+        rc = main(["--whoami", "--api-key", "k", "--server", "https://w.example"])
+        assert rc == 20
+        assert "[session_api_failed]" in capsys.readouterr().err