Add max reasoning effort option for Opus 4.6 models (#322)

The codebase already has full backend support for "max" effort in types,
validation, SDK passthrough, and error handling. The only missing piece
was the `CLAUDE_THINKING_OPTIONS` array, which didn't surface "max" in
the UI dropdown.

Fixes #271

Co-authored-by: Claude Code <noreply@anthropic.com>

packages/server/src/server/agent/providers/claude/claude-models.ts

@@ -4,6 +4,7 @@ const CLAUDE_THINKING_OPTIONS = [
   { id: "low", label: "Low" },
   { id: "medium", label: "Medium" },
   { id: "high", label: "High" },
+  { id: "max", label: "Max" },
 ] as const;
 
 const CLAUDE_MODELS: AgentModelDefinition[] = [