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/adapter.py

@@ -321,6 +321,24 @@ def _tool_end_fields(tool_name: str) -> dict[str, Any]:
     return fields
 
 
+def _tool_emoji(tool_name: str) -> str | None:
+    """Cosmetic per-tool emoji for the ``tool.start`` frame.
+
+    Resolved via hermes' own display layer (``agent.display.get_tool_emoji``):
+    active-skin ``tool_emojis`` overrides first, then the tool registry's
+    per-tool ``emoji`` field — so icons track the user's hermes theme and
+    new/plugin tools get their registered glyph for free. Returns ``None``
+    when the tool is unknown (or the import fails) so the frame omits the
+    field and the app falls back to its own default glyph.
+    """
+    try:
+        from agent.display import get_tool_emoji
+
+        return get_tool_emoji(tool_name, default="") or None
+    except Exception:
+        return None
+
+
 # ---------------------------------------------------------------------------
 # Runtime-metadata footer (post_api_request hook)
 #
@@ -1569,6 +1587,7 @@ class AndroidAdapter(BasePlatformAdapter):
                     name,
                     preview=preview,
                     args=args,
+                    emoji=_tool_emoji(name),
                     thread_id=thread_id,
                 ),
             )