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.

cli.py

@@ -5320,6 +5320,28 @@ class HermesCLI:
                 message if isinstance(message, str) else "", images
             )
 
+        # Expand @ context references (e.g. @file:main.py, @diff, @folder:src/)
+        if isinstance(message, str) and "@" in message:
+            try:
+                from agent.context_references import preprocess_context_references
+                from agent.model_metadata import get_model_context_length
+                _ctx_len = get_model_context_length(
+                    self.model, base_url=self.base_url or "", api_key=self.api_key or "")
+                _ctx_result = preprocess_context_references(
+                    message, cwd=os.getcwd(), context_length=_ctx_len)
+                if _ctx_result.expanded or _ctx_result.blocked:
+                    if _ctx_result.references:
+                        _cprint(
+                            f"  {_DIM}[@ context: {len(_ctx_result.references)} ref(s), "
+                            f"{_ctx_result.injected_tokens} tokens]{_RST}")
+                    for w in _ctx_result.warnings:
+                        _cprint(f"  {_DIM}⚠ {w}{_RST}")
+                    if _ctx_result.blocked:
+                        return "\n".join(_ctx_result.warnings) or "Context injection refused."
+                    message = _ctx_result.message
+            except Exception as e:
+                logging.debug("@ context reference expansion failed: %s", e)
+
         # Add user message to history
         self.conversation_history.append({"role": "user", "content": message})