fix(photon): support E.164 and DM GUID targets for home channel

Allow PHOTON_HOME_CHANNEL to accept a bare E.164 phone number or a
`any;-;+1...` DM chat GUID in addition to a Spectrum space id. Inbound
DM spaces are cached so replies resolve without a second SDK lookup,
and `photon` is added to _PHONE_PLATFORMS so send_message treats E.164
strings as explicit targets rather than falling through to channel-name
resolution.

tests/tools/test_send_message_tool.py

@@ -1199,6 +1199,11 @@ class TestParseTargetRefE164:
         assert chat_id == "+15551234567"
         assert is_explicit is True
 
+    def test_photon_e164_is_explicit(self):
+        chat_id, _, is_explicit = _parse_target_ref("photon", "+15551234567")
+        assert chat_id == "+15551234567"
+        assert is_explicit is True
+
     def test_signal_bare_digits_still_work(self):
         """Bare digit strings continue to match the generic numeric branch."""
         chat_id, _, is_explicit = _parse_target_ref("signal", "15551234567")