feat(tools): centralize tool emoji metadata in registry + skin integration

- Add 'emoji' field to ToolEntry and 'get_emoji()' to ToolRegistry
- Add emoji= to all 50+ registry.register() calls across tool files
- Add get_tool_emoji() helper in agent/display.py with 3-tier resolution:
  skin override → registry default → hardcoded fallback
- Replace hardcoded emoji maps in run_agent.py, delegate_tool.py, and
  gateway/run.py with centralized get_tool_emoji() calls
- Add 'tool_emojis' field to SkinConfig so skins can override per-tool
  emojis (e.g. ares skin could use swords instead of wrenches)
- Add 11 tests (5 registry emoji, 6 display/skin integration)
- Update AGENTS.md skin docs table

Based on the approach from PR #1061 by ForgingAlex (emoji centralization
in registry). This salvage fixes several issues from the original:
- Does NOT split the cronjob tool (which would crash on missing schemas)
- Does NOT change image_generate toolset/requires_env/is_async
- Does NOT delete existing tests
- Completes the centralization (gateway/run.py was missed)
- Hooks into the skin system for full customizability

tests/tools/test_registry.py

@@ -232,6 +232,48 @@ class TestCheckFnExceptionHandling:
         assert any(u["name"] == "crashes" for u in unavailable)
 
 
+class TestEmojiMetadata:
+    """Verify per-tool emoji registration and lookup."""
+
+    def test_emoji_stored_on_entry(self):
+        reg = ToolRegistry()
+        reg.register(
+            name="t", toolset="s", schema=_make_schema(),
+            handler=_dummy_handler, emoji="🔥",
+        )
+        assert reg._tools["t"].emoji == "🔥"
+
+    def test_get_emoji_returns_registered(self):
+        reg = ToolRegistry()
+        reg.register(
+            name="t", toolset="s", schema=_make_schema(),
+            handler=_dummy_handler, emoji="🎯",
+        )
+        assert reg.get_emoji("t") == "🎯"
+
+    def test_get_emoji_returns_default_when_unset(self):
+        reg = ToolRegistry()
+        reg.register(
+            name="t", toolset="s", schema=_make_schema(),
+            handler=_dummy_handler,
+        )
+        assert reg.get_emoji("t") == "⚡"
+        assert reg.get_emoji("t", default="🔧") == "🔧"
+
+    def test_get_emoji_returns_default_for_unknown_tool(self):
+        reg = ToolRegistry()
+        assert reg.get_emoji("nonexistent") == "⚡"
+        assert reg.get_emoji("nonexistent", default="❓") == "❓"
+
+    def test_emoji_empty_string_treated_as_unset(self):
+        reg = ToolRegistry()
+        reg.register(
+            name="t", toolset="s", schema=_make_schema(),
+            handler=_dummy_handler, emoji="",
+        )
+        assert reg.get_emoji("t") == "⚡"
+
+
 class TestSecretCaptureResultContract:
     def test_secret_request_result_does_not_include_secret_value(self):
         result = {