feat: raise default tool-calling iteration limit from 90 to 500

The default max_iterations/agent.max_turns budget was set when long
agentic runs were rare; complex tasks now routinely exceed 90 tool
calls. Raise the default to 500 across every surface that hardcodes
the fallback: AIAgent constructor, DEFAULT_CONFIG, CLI resolution
chain, gateway env bridge, cron scheduler, and TUI gateway. Explicit
user config values are unaffected (deep-merge preserves them; no
_config_version bump needed).

Docs (en + zh-Hans), CLI help text, tips, and pinned tests updated
to match.

AGENTS.md

@@ -325,7 +325,7 @@ class AIAgent:
         provider: str = None,
         api_mode: str = None,              # "chat_completions" | "codex_responses" | ...
         model: str = "",                   # empty → resolved from config/provider later
-        max_iterations: int = 90,          # tool-calling iterations (shared with subagents)
+        max_iterations: int = 500,         # tool-calling iterations (shared with subagents)
         enabled_toolsets: list = None,
         disabled_toolsets: list = None,
         quiet_mode: bool = False,