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.

cli.py

@@ -508,7 +508,18 @@ def _get_available_skills() -> Dict[str, List[str]]:
     return skills_by_category
 
 
-def build_welcome_banner(console: Console, model: str, cwd: str, tools: List[dict] = None, enabled_toolsets: List[str] = None, session_id: str = None):
+def _format_context_length(tokens: int) -> str:
+    """Format a token count for display (e.g. 128000 → '128K', 1048576 → '1M')."""
+    if tokens >= 1_000_000:
+        val = tokens / 1_000_000
+        return f"{val:g}M"
+    elif tokens >= 1_000:
+        val = tokens / 1_000
+        return f"{val:g}K"
+    return str(tokens)
+
+
+def build_welcome_banner(console: Console, model: str, cwd: str, tools: List[dict] = None, enabled_toolsets: List[str] = None, session_id: str = None, context_length: int = None):
     """
     Build and print a Claude Code-style welcome banner with caduceus on left and info on right.
     
@@ -519,6 +530,7 @@ def build_welcome_banner(console: Console, model: str, cwd: str, tools: List[dic
         tools: List of tool definitions
         enabled_toolsets: List of enabled toolset names
         session_id: Unique session identifier for logging
+        context_length: Model's context window size in tokens
     """
     from model_tools import check_tool_availability, TOOLSET_REQUIREMENTS
     
@@ -544,7 +556,8 @@ def build_welcome_banner(console: Console, model: str, cwd: str, tools: List[dic
     if len(model_short) > 28:
         model_short = model_short[:25] + "..."
     
-    left_lines.append(f"[#FFBF00]{model_short}[/] [dim #B8860B]·[/] [dim #B8860B]Nous Research[/]")
+    ctx_str = f" [dim #B8860B]·[/] [dim #B8860B]{_format_context_length(context_length)} context[/]" if context_length else ""
+    left_lines.append(f"[#FFBF00]{model_short}[/]{ctx_str} [dim #B8860B]·[/] [dim #B8860B]Nous Research[/]")
     left_lines.append(f"[dim #B8860B]{cwd}[/]")
     
     # Add session ID if provided
@@ -1079,6 +1092,11 @@ class HermesCLI:
             # Get terminal working directory (where commands will execute)
             cwd = os.getenv("TERMINAL_CWD", os.getcwd())
             
+            # Get context length for display
+            ctx_len = None
+            if hasattr(self, 'agent') and self.agent and hasattr(self.agent, 'context_compressor'):
+                ctx_len = self.agent.context_compressor.context_length
+            
             # Build and display the banner
             build_welcome_banner(
                 console=self.console,
@@ -1087,6 +1105,7 @@ class HermesCLI:
                 tools=tools,
                 enabled_toolsets=self.enabled_toolsets,
                 session_id=self.session_id,
+                context_length=ctx_len,
             )
         
         # Show tool availability warnings if any tools are disabled