fix(core): mark prompt-cache breakpoints for Claude models

[tjholm]

Fixes SUGA-1955

Breakpoints

• System text (covers the tools), the end of a turn's history, and each step's newest message
• The history mark is PromptMessage.cacheBreakpoint, set by modelPrompt before the trailing [Turn] message; kept in the turn checkpoint
• The step mark moves rather than accumulates, since the SDK carries prepareStep messages forward and Anthropic allows four

Providers

• ProviderDialect.cacheBreakpoint: Anthropic always, OpenRouter for anthropic/* models
• Everyone else's requests are unchanged

Checked locally (OpenRouter, claude-haiku-5.5)

• Later turns read 95–99% of the prompt from cache; cost per turn about 1/6 of uncached
• A turn after one with tool calls re-writes a few hundred tokens of history; left for a follow-up

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	sugabots-website	f55bf08	Oct 09 2026, 02:34 AM