Cache-stable request assembly

commerce_common/ is commerce-common/commerce_common/, shopping_agent/ is shopping-agent/core/shopping_agent/, and merchant_agent/ is merchant-agent/core/merchant_agent/.

The three breakpoints

A request's cacheable prefix runs tools, then system, then the messages; a breakpoint ends a span the next call reads back. The reference agents place three, all from commerce_common/prompt_assembly.py:

1. The last tool: with_tool_cache_control(tools).
2. The static system text: build_system_blocks(static, context) marks it and appends the context block unmarked.
3. The newest persisted message: build_request_messages(messages, rolling_breakpoint=...) marks the last persisted block on the outgoing request only; the next call then reads the earlier rounds, search payloads included, from cache.

Everything per request goes in the context block, which the role's build_dynamic_context(...) renders once per turn. In the static block or the tool list, those bytes would break the system or tool span on every request; in the context block they cost one re-read of the conversation on a turn whose cart, page, or facts moved, and the rolling marker holds from that turn's next round. The clock renders to the hour (context_clock), so a new minute moves nothing.