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

docs/protocol/frames.schema.json

@@ -47,7 +47,7 @@
       "message.stop": { "payload": { "message_id": { "type": "string" }, "final_text": { "type": "string" }, "reasoning": { "type": "string" }, "model": { "type": "string" }, "tokens": { "type": "integer" }, "runtime": { "$ref": "#/definitions/runtime" }, "ts": { "type": "integer" } } },
       "message.deleted": { "description": "The given message(s) were deleted from a chat/thread. Response to a message.delete request (id set) and broadcast to every device so all drop them from their cache; also outboxed so an offline device learns of the deletion on its next sync.", "payload": { "message_ids": { "type": "array", "items": { "type": "string" } } } },
       "commentary": { "description": "Intermediate assistant beat.", "payload": { "message_id": { "type": "string" }, "text": { "type": "string" } } },
-      "tool.start": { "payload": { "index": { "type": "integer" }, "name": { "type": "string" }, "preview": { "type": "string" }, "args": { "type": "object" } } },
+      "tool.start": { "description": "Cosmetic per-tool emoji (resolved server-side via hermes' get_tool_emoji: active-skin overrides, then the tool registry's per-tool emoji); omitted when the tool is unknown so the app falls back to its own default glyph.", "payload": { "index": { "type": "integer" }, "name": { "type": "string" }, "preview": { "type": "string" }, "args": { "type": "object" }, "emoji": { "type": "string" } } },
       "tool.progress": { "payload": { "index": { "type": "integer" }, "name": { "type": "string" }, "note": { "type": "string" } } },
       "tool.end": { "payload": { "index": { "type": "integer" }, "name": { "type": "string" }, "ok": { "type": "boolean" }, "duration": { "type": "number" }, "output_preview": { "type": "string" } } },
       "typing": { "payload": { "on": { "type": "boolean" } } },