computeNonMessageBreakdown estimated Skills tokens from the unfiltered
session.skills registry and subtracted that from the first system-prompt
block, which only ever contained the rendered (filtered) skills. Hidden
explicit-only skills (hide/disable-model-invocation) and all skills when
the read tool was absent inflated Skills and clamped System prompt to 0.
Add a renderedSkills helper mirroring buildSystemPrompt's filter and use
it for the Skills estimate so the category split matches the provider
prompt.
Fixes#6498
roboomp
e9ae836a46