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/04-wire-protocol.md

@@ -126,7 +126,7 @@ truncated / nothing).
 
 ```json
 {"type":"tool.start","chat_id":"…","payload":{
-   "index":3,"name":"terminal","preview":"pytest -q","args":{"command":"pytest -q"}}}
+   "index":3,"name":"terminal","preview":"pytest -q","args":{"command":"pytest -q"},"emoji":"💻"}}
 {"type":"tool.progress","chat_id":"…","payload":{"index":3,"name":"terminal","note":"running…"}}
 {"type":"tool.end","chat_id":"…","payload":{"index":3,"name":"terminal","ok":true,"duration":12.4,
    "output_preview":"12 passed"}}
@@ -135,6 +135,11 @@ truncated / nothing).
 `args` may be large; the app truncates per its setting. `output_preview` is a
 short tail (full output is not streamed — it lives in agent history).
 
+`emoji` is a cosmetic per-tool glyph resolved server-side via hermes'
+`get_tool_emoji` (active-skin overrides, then the tool registry's per-tool
+`emoji` — e.g. `read_file` 📖, `write_file` ✍️, `terminal` 💻). Omitted when
+the tool is unknown, so the app falls back to its own default glyph.
+
 ### `typing` / `typing.stop`
 
 ```json