Send `system` as a structured text block with an ephemeral cache_control
breakpoint and cache the last tool schema, so multi-round agent runs read
the stable system+tools prefix from cache instead of re-billing it. Gate
the system breakpoint so tiny tool-less prompts skip the cache-write
premium. Log cache_read/creation tokens at message_start.
Fixes#791
Co-authored-by: Ethan <23321960+0xLeathery@users.noreply.github.com>
