feat(models): surface and prevent silent max-tokens truncation

- Raise --max-tokens default from 4096 to 8192.
- Auto-raise MaxTokens toward the model's catalog Limit.Output (capped at
  32768) when the user hasn't set --max-tokens explicitly and no per-model
  modelSettings override applied. Prevents silent 4k/8k truncation on
  models that support 32k-262k output.
- Surface FinishReasonLength at turn end: the app now subscribes to
  TurnEndEvent and renders a system-message banner explaining the current
  cap, the model's known ceiling, and how to raise it. Previously the TUI
  swallowed 'length' stops, producing 'ghost' truncations.
- Export FinishReason* constants on pkg/kit (Stop, Length, ToolCalls,
  ContentFilter, Error, Other, Unknown) and fix stale comments that used
  Anthropic-style strings.
- Add Kit.MaxTokens() and Kit.MaxOutputLimit() SDK accessors, backed by
  Agent.GetMaxTokens() which correctly returns 0 for providers that
  suppress the param (e.g. Codex OAuth).
- Tests: rightSizeMaxTokens covers 7 paths (cap, raise, preserve,
  explicit flag, nil info, zero limit); handleTurnEnd covers length/
  non-length/nil-sendFn and the fallback message formatter.
- Docs: update configuration.md, cli/flags.md, and kit-extensions skill
  to reflect the new default and behavior.

skills/kit-extensions/SKILL.md

@@ -93,7 +93,7 @@ api.OnAgentEnd(func(e ext.AgentEndEvent, ctx ext.Context) {
     // e.Response string
     // e.StopReason string — "error" (on failure), "completed" (when LLM returns
     //   empty stop reason), or the raw LLM provider value passed through
-    //   (e.g. "stop", "end_turn", "max_tokens", "tool_use").
+    //   (e.g. "stop", "length" (max output tokens hit), "tool-calls", "content-filter").
     //   To detect errors, check e.StopReason == "error".
     //   Do NOT compare against "completed" for success — instead check != "error".
 })