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_egress.py

@@ -80,11 +80,19 @@ class TestAgentProviderHostCredentials(unittest.TestCase):
                 "forward_host_credentials": "yes",
             })
 
-    def test_forward_host_credentials_rejected_for_claude(self):
+    def test_forward_host_credentials_allowed_for_claude(self):
+        b = _provider_config_bottle({
+            "template": "claude",
+            "forward_host_credentials": True,
+        })
+        self.assertTrue(b.agent_provider.forward_host_credentials)
+
+    def test_forward_host_credentials_and_auth_token_rejected_together(self):
         with self.assertRaises(ManifestError):
             _provider_config_bottle({
                 "template": "claude",
                 "forward_host_credentials": True,
+                "auth_token": "SOME_TOKEN",
             })
 
     def test_auth_token_defaults_empty(self):