feat: smart context length probing with persistent caching + banner display

Replaces the unsafe 128K fallback for unknown models with a descending
probe strategy (2M → 1M → 512K → 200K → 128K → 64K → 32K). When a
context-length error occurs, the agent steps down tiers and retries.
The discovered limit is cached per model+provider combo in
~/.hermes/context_length_cache.yaml so subsequent sessions skip probing.

Also parses API error messages to extract the actual context limit
(e.g. 'maximum context length is 32768 tokens') for instant resolution.

The CLI banner now displays the context window size next to the model
name (e.g. 'claude-opus-4 · 200K context · Nous Research').

Changes:
- agent/model_metadata.py: CONTEXT_PROBE_TIERS, persistent cache
  (save/load/get), parse_context_limit_from_error(), get_next_probe_tier()
- agent/context_compressor.py: accepts base_url, passes to metadata
- run_agent.py: step-down logic in context error handler, caches on success
- cli.py + hermes_cli/banner.py: context length in welcome banner
- tests: 22 new tests for probing, parsing, and caching

Addresses #132. PR #319's approach (8K default) rejected — too conservative.

agent/context_compressor.py

@@ -34,17 +34,20 @@ class ContextCompressor:
         summary_target_tokens: int = 2500,
         quiet_mode: bool = False,
         summary_model_override: str = None,
+        base_url: str = "",
     ):
         self.model = model
+        self.base_url = base_url
         self.threshold_percent = threshold_percent
         self.protect_first_n = protect_first_n
         self.protect_last_n = protect_last_n
         self.summary_target_tokens = summary_target_tokens
         self.quiet_mode = quiet_mode
 
-        self.context_length = get_model_context_length(model)
+        self.context_length = get_model_context_length(model, base_url=base_url)
         self.threshold_tokens = int(self.context_length * threshold_percent)
         self.compression_count = 0
+        self._context_probed = False  # True after a step-down from context error
 
         self.last_prompt_tokens = 0
         self.last_completion_tokens = 0