feat: prefer-tools system prompt + comprehensive test suite (95 unit + 10 E2E tests)

fix: tool-intent classifier routes translation/calc/convert/datetime/dictionary to tool pipeline

feat: add 6 new tools (calculator, wikipedia, dictionary, unit converter, datetime, translation) + smart key rotation + direct answer fast-path + tools on all steps

feat: Planner-Executor AI agent with autonomous task execution, memory, and weather tool