* feat: add contextBudgetTokens config field for tool-loop trimming
* feat: implement _trim_history_for_budget for tool-loop cost reduction
* feat: thread contextBudgetTokens into AgentLoop constructor
* feat: wire context budget trimming into agent loop
* refactor: move trim_history_for_budget to helpers and add docs
- Extract trim_history_for_budget() as a pure function in helpers.py
- AgentLoop._trim_history_for_budget becomes a thin wrapper
- Add docs/CONTEXT_BUDGET.md with usage guide and trade-off notes
- Replace wrapper tests with direct helper unit tests
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Co-authored-by: chengyongru <chengyongru.ai@gmail.com>