feat: complete plugin platform parity — all 12 integration points

Extends the platform plugin interface from Phase 1 to cover every
touchpoint where built-in platforms have hardcoded behavior.

- allowed_users_env / allow_all_env: per-platform auth env vars
- max_message_length: smart-chunking for send_message tool
- pii_safe: session PII redaction flag
- emoji: CLI/gateway display
- allow_update_command: /update access control

send_message tool (tools/send_message_tool.py):
- Replaced hardcoded platform_map dict with Platform() call
- Added _send_via_adapter() for plugin platforms — routes through
  live gateway adapter when available
- Registry-aware max message length for smart chunking

Cron delivery (cron/scheduler.py):
- Replaced hardcoded 15-entry platform_map with Platform() call
- Plugin platforms now work as cron delivery targets

User authorization (gateway/run.py _is_user_authorized):
- Registry fallback: checks PlatformEntry.allowed_users_env and
  allow_all_env when platform not in hardcoded maps
- Plugin platforms get per-platform auth support

_UPDATE_ALLOWED_PLATFORMS: checks registry allow_update_command flag
Channel directory: includes plugin platforms in session enumeration
Orphaned config warning: descriptive message when plugin platform is
  in config but no plugin registered it
Gateway weakref: _gateway_runner_ref for cross-module adapter access

hermes status: shows plugin platforms with (plugin) tag
hermes gateway setup: plugin platforms appear in menu with setup hints
hermes_cli/platforms.py: get_all_platforms() merges with registry,
  platform_label() falls back to registry for plugin names

- 8 new tests (extended fields, cron resolution, platforms merge)
- Updated 3 tests for new Platform() based resolution
- 2829 passed, 24 pre-existing failures, zero new failures

tests/gateway/test_platform_registry.py

@@ -265,3 +265,113 @@ class TestGatewayConfigPluginPlatform:
             assert "badconfig" not in connected_values
         finally:
             _reg.unregister("badconfig")
+
+
+# ── Extended PlatformEntry fields ─────────────────────────────────────
+
+
+class TestPlatformEntryExtendedFields:
+    """Test the auth, message length, and display fields on PlatformEntry."""
+
+    def test_default_field_values(self):
+        entry = PlatformEntry(
+            name="test",
+            label="Test",
+            adapter_factory=lambda cfg: None,
+            check_fn=lambda: True,
+        )
+        assert entry.allowed_users_env == ""
+        assert entry.allow_all_env == ""
+        assert entry.max_message_length == 0
+        assert entry.pii_safe is False
+        assert entry.emoji == "🔌"
+        assert entry.allow_update_command is True
+
+    def test_custom_auth_fields(self):
+        entry = PlatformEntry(
+            name="irc",
+            label="IRC",
+            adapter_factory=lambda cfg: None,
+            check_fn=lambda: True,
+            allowed_users_env="IRC_ALLOWED_USERS",
+            allow_all_env="IRC_ALLOW_ALL_USERS",
+            max_message_length=450,
+            pii_safe=False,
+            emoji="💬",
+        )
+        assert entry.allowed_users_env == "IRC_ALLOWED_USERS"
+        assert entry.allow_all_env == "IRC_ALLOW_ALL_USERS"
+        assert entry.max_message_length == 450
+        assert entry.emoji == "💬"
+
+
+# ── Cron platform resolution ─────────────────────────────────────────
+
+
+class TestCronPlatformResolution:
+    """Test that cron delivery accepts plugin platform names."""
+
+    def test_builtin_platform_resolves(self):
+        """Built-in platform names resolve via Platform() call."""
+        p = Platform("telegram")
+        assert p is Platform.TELEGRAM
+
+    def test_plugin_platform_resolves(self):
+        """Plugin platform names create dynamic enum members."""
+        p = Platform("irc")
+        assert p.value == "irc"
+
+    def test_invalid_platform_type_rejected(self):
+        """Non-string values are still rejected."""
+        with pytest.raises(ValueError):
+            Platform(None)
+
+
+# ── platforms.py integration ──────────────────────────────────────────
+
+
+class TestPlatformsMerge:
+    """Test get_all_platforms() merges with registry."""
+
+    def test_get_all_platforms_includes_builtins(self):
+        from hermes_cli.platforms import get_all_platforms, PLATFORMS
+        merged = get_all_platforms()
+        for key in PLATFORMS:
+            assert key in merged
+
+    def test_get_all_platforms_includes_plugin(self):
+        from hermes_cli.platforms import get_all_platforms
+        from gateway.platform_registry import platform_registry as _reg
+
+        _reg.register(PlatformEntry(
+            name="testmerge",
+            label="TestMerge",
+            adapter_factory=lambda cfg: None,
+            check_fn=lambda: True,
+            source="plugin",
+            emoji="🧪",
+        ))
+        try:
+            merged = get_all_platforms()
+            assert "testmerge" in merged
+            assert "TestMerge" in merged["testmerge"].label
+        finally:
+            _reg.unregister("testmerge")
+
+    def test_platform_label_plugin_fallback(self):
+        from hermes_cli.platforms import platform_label
+        from gateway.platform_registry import platform_registry as _reg
+
+        _reg.register(PlatformEntry(
+            name="labeltest",
+            label="LabelTest",
+            adapter_factory=lambda cfg: None,
+            check_fn=lambda: True,
+            source="plugin",
+            emoji="🏷️",
+        ))
+        try:
+            label = platform_label("labeltest")
+            assert "LabelTest" in label
+        finally:
+            _reg.unregister("labeltest")