feat: add Anthropic Fast Mode support to /fast command (#7037)

Extends the /fast command to support Anthropic's Fast Mode beta in addition
to OpenAI Priority Processing. When enabled on Claude Opus 4.6, adds
speed:"fast" and the fast-mode-2026-02-01 beta header to API requests for
~2.5x faster output token throughput.

Changes:
- hermes_cli/models.py: Add _ANTHROPIC_FAST_MODE_MODELS registry,
  model_supports_fast_mode() now recognizes Claude Opus 4.6,
  resolve_fast_mode_overrides() returns {speed: fast} for Anthropic
  vs {service_tier: priority} for OpenAI
- agent/anthropic_adapter.py: Add _FAST_MODE_BETA constant,
  build_anthropic_kwargs() accepts fast_mode=True which injects
  speed:fast + beta header via extra_headers (skipped for third-party
  Anthropic-compatible endpoints like MiniMax)
- run_agent.py: Pass fast_mode to build_anthropic_kwargs in the
  anthropic_messages path of _build_api_kwargs()
- cli.py: Update _handle_fast_command with provider-aware messaging
  (shows 'Anthropic Fast Mode' vs 'Priority Processing')
- hermes_cli/commands.py: Update /fast description to mention both
  providers
- tests: 13 new tests covering Anthropic model detection, override
  resolution, CLI availability, routing, adapter kwargs, and
  third-party endpoint safety

run_agent.py

@@ -5466,6 +5466,7 @@ class AIAgent:
                 preserve_dots=self._anthropic_preserve_dots(),
                 context_length=ctx_len,
                 base_url=getattr(self, "_anthropic_base_url", None),
+                fast_mode=self.request_overrides.get("speed") == "fast",
             )
 
         if self.api_mode == "codex_responses":