fix: route the model to br_coverage from INSTRUCTIONS

A registered tool the instructions never mention is discoverable but not
routed. The tool shipped in the previous release; the instructions did not
tell the model when to reach for it, so the behaviour change was only half
delivered.

INSTRUCTIONS now carry the rule: when a search comes back empty, or the
question touches material this connector does not hold, call the coverage
tool and relay what it says is missing. Absence here is not absence in the
law.

Caught by boutique-mcp's drift test, which asserts every registered tool is
mentioned in INSTRUCTIONS - the stricter direction the rest of the fleet
does not check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>