feat(claude): add forward_host_credentials support

Reads the host's Claude OAuth session key from ~/.claude.json at launch
and forwards it only to the egress sidecar (never to the agent), placing
a placeholder CLAUDE_CODE_OAUTH_TOKEN in the agent env so Claude Code
starts without seeing the real credential.

Mirrors the existing Codex forward_host_credentials flow (PRD 0029).
Adds claude_auth.py to extract and validate the sessionKey, a
CLAUDE_HOST_CREDENTIAL_TOKEN_REF constant in egress.py, and updates
manifest_agent.py to allow the flag for both 'codex' and 'claude'
templates. Also adds a mutual-exclusion check that rejects setting
both auth_token and forward_host_credentials together.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_manifest_validation.py

@@ -82,10 +82,22 @@ class TestAgentProviderValidation(unittest.TestCase):
                 "b", {"forward_host_credentials": True, "template": "weird"}
             )
 
-    def test_forward_creds_non_codex_template(self) -> None:
+    def test_forward_creds_pi_template_rejected(self) -> None:
         with self.assertRaises(ManifestError):
             ManifestAgentProvider.from_dict(
-                "b", {"forward_host_credentials": True, "template": "claude"}
+                "b", {"forward_host_credentials": True, "template": "pi"}
+            )
+
+    def test_forward_creds_claude_allowed(self) -> None:
+        p = ManifestAgentProvider.from_dict(
+            "b", {"forward_host_credentials": True, "template": "claude"}
+        )
+        self.assertTrue(p.forward_host_credentials)
+
+    def test_forward_creds_and_auth_token_rejected(self) -> None:
+        with self.assertRaises(ManifestError):
+            ManifestAgentProvider.from_dict(
+                "b", {"forward_host_credentials": True, "auth_token": "T", "template": "claude"}
             )
 
     def test_valid_claude_auth_token(self) -> None: