feat: capture provider rate limit headers and show in /usage (#6541)

Parse x-ratelimit-* headers from inference API responses (Nous Portal,
OpenRouter, OpenAI-compatible) and display them in the /usage command.

- New agent/rate_limit_tracker.py: parse 12 rate limit headers (RPM/RPH/
  TPM/TPH limits, remaining, reset timers), format as progress bars (CLI)
  or compact one-liner (gateway)
- Hook into streaming path in run_agent.py: stream.response.headers is
  available on the OpenAI SDK Stream object before chunks are consumed
- CLI /usage: appends rate limit section with progress bars + warnings
  when any bucket exceeds 80%
- Gateway /usage: appends compact rate limit summary
- 24 unit tests covering parsing, formatting, edge cases

Headers captured per response:
  x-ratelimit-{limit,remaining,reset}-{requests,tokens}{,-1h}

Example CLI display:
  Nous Rate Limits (captured just now):
    Requests/min [░░░░░░░░░░░░░░░░░░░░]  0.1%  1/800 used  (799 left, resets in 59s)
    Tokens/hr    [░░░░░░░░░░░░░░░░░░░░]  0.0%  49/336.0M   (336.0M left, resets in 52m)

gateway/run.py

@@ -5280,19 +5280,28 @@ class GatewayRunner:
 
         agent = self._running_agents.get(session_key)
         if agent and hasattr(agent, "session_total_tokens") and agent.session_api_calls > 0:
-            lines = [
-                "📊 **Session Token Usage**",
-                f"Prompt (input): {agent.session_prompt_tokens:,}",
-                f"Completion (output): {agent.session_completion_tokens:,}",
-                f"Total: {agent.session_total_tokens:,}",
-                f"API calls: {agent.session_api_calls}",
-            ]
+            lines = []
+
+            # Rate limits first (when available from provider headers)
+            rl_state = agent.get_rate_limit_state()
+            if rl_state and rl_state.has_data:
+                from agent.rate_limit_tracker import format_rate_limit_compact
+                lines.append(f"⏱️ **Rate Limits:** {format_rate_limit_compact(rl_state)}")
+                lines.append("")
+
+            # Session token usage
+            lines.append("📊 **Session Token Usage**")
+            lines.append(f"Prompt (input): {agent.session_prompt_tokens:,}")
+            lines.append(f"Completion (output): {agent.session_completion_tokens:,}")
+            lines.append(f"Total: {agent.session_total_tokens:,}")
+            lines.append(f"API calls: {agent.session_api_calls}")
             ctx = agent.context_compressor
             if ctx.last_prompt_tokens:
                 pct = min(100, ctx.last_prompt_tokens / ctx.context_length * 100) if ctx.context_length else 0
                 lines.append(f"Context: {ctx.last_prompt_tokens:,} / {ctx.context_length:,} ({pct:.0f}%)")
             if ctx.compression_count:
                 lines.append(f"Compressions: {ctx.compression_count}")
+
             return "\n".join(lines)
 
         # No running agent -- check session history for a rough count