ask_inputs: ask for blockers, not permission

Real-world report: asked to "read the first line of this, then pause", the
assistant called ask_inputs to ask "Would you like me to read only the
first line and then stop?" - a round trip spent restating the instruction
it had just been given.

All three places that instructed on this tool were purely permissive. Each
said when to ask ("decisions, clarifications, or document uploads",
"clarify a missing premise", "would materially change the work"); none said
when not to, and "clarification" is an open invitation.

Whether to ask is judgment, so it stays prose rather than becoming a guard.
The instruction now draws the line lawyers already work to - blocker versus
deferrable: an instruction only the user can give, or a document never
provided, versus ambiguity you resolve on the most reasonable reading and
flag as an assumption. Plus the one hard prohibition the failure needed:
never seek confirmation of an instruction already given.

Same wording across the tool description (read at the point of use) and
both system prompts, so the three cannot drift apart again.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018bhP89cYSdEEjHNPnxacns