Three separate Claude Code bugs were silently destroying prompt-cache reuse for subagents

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2.1.265 fixes resuming a foreground-spawned subagent changing its tool list and system prompt prefix, agent teammates and resumed subagents moving SubagentStart hook context and preloaded skills out of the prompt prefix on later turns, and both are called out explicitly as breaking prompt-cache reuse. A third fix keeps a resume after the process died mid-tool from rewriting the last prompt. Anyone running multi-agent or long-resumed sessions was paying full uncached input token cost on every turn after a resume without any visible error, which is the expensive kind of bug because nothing fails.