Per-tool emoji on tool cards (gateway-resolved via hermes get_tool_emoji)

tool.start gains an optional cosmetic 'emoji' field, resolved server-side
through hermes' own display layer (active-skin overrides, then the tool
registry's per-tool emoji) so icons track the user's hermes theme and
new/plugin tools get their glyph for free. Omitted for unknown tools so
the app falls back to its default wrench.

- protocol.py: tool_start(emoji=...) kwarg, payload field when set
- adapter.py: _tool_emoji() helper (lazy import, None on unknown/failure)
- frames.schema.json + docs/04: field documented
- app: ToolStartPayload.emoji -> ToolItem.emoji -> ToolCard header
- tests: frame shape, resolution/fallback, end-to-end tool.start emoji

gateway-plugin/protocol.py

@@ -423,12 +423,15 @@ def tool_start(
     thread_id: str | None = None,
     preview: str | None = None,
     args: dict[str, Any] | None = None,
+    emoji: str | None = None,
 ) -> Frame:
     payload: dict[str, Any] = {"index": index, "name": name}
     if preview:
         payload["preview"] = preview
     if args:
         payload["args"] = args
+    if emoji:
+        payload["emoji"] = emoji
     return Frame(
         type=TYPE_TOOL_START,
         chat_id=chat_id,