feat: @ context references — inline file, folder, diff, git, and URL injection

Add @file:path, @folder:dir, @diff, @staged, @git:N, and @url:
references that expand inline before the message reaches the LLM.
Supports line ranges (@file:main.py:10-50), token budget enforcement
(soft warn at 25%, hard block at 50%), and path sandboxing for gateway.

Core module from PR #2090 by @kshitijk4poor. CLI and gateway wiring
rewritten against current main. Fixed asyncio.run() crash when called
from inside a running event loop (gateway).

Closes #682.

gateway/run.py

@@ -2058,7 +2058,31 @@ class GatewayRunner:
                 "message": message_text[:500],
             }
             await self.hooks.emit("agent:start", hook_ctx)
-            
+
+            # Expand @ context references (@file:, @folder:, @diff, etc.)
+            if "@" in message_text:
+                try:
+                    from agent.context_references import preprocess_context_references_async
+                    from agent.model_metadata import get_model_context_length
+                    _msg_cwd = os.environ.get("MESSAGING_CWD", os.path.expanduser("~"))
+                    _msg_ctx_len = get_model_context_length(
+                        self._model, base_url=self._base_url or "")
+                    _ctx_result = await preprocess_context_references_async(
+                        message_text, cwd=_msg_cwd,
+                        context_length=_msg_ctx_len, allowed_root=_msg_cwd)
+                    if _ctx_result.blocked:
+                        _adapter = self.adapters.get(source.platform)
+                        if _adapter:
+                            await _adapter.send(
+                                source.chat_id,
+                                "\n".join(_ctx_result.warnings) or "Context injection refused.",
+                            )
+                        return
+                    if _ctx_result.expanded:
+                        message_text = _ctx_result.message
+                except Exception as exc:
+                    logger.debug("@ context reference expansion failed: %s", exc)
+
             # Run the agent
             agent_result = await self._run_agent(
                 message=message_text,