feat(pm): flag decision-box cards with open downstream dependents

Standing triage instruction ④ (2026-08-11 maintainer ruling) asks every
round-closing brief to flag the `needs-user-decision` cards some open card's
`Blocked-by:` line points at. That was measured unrunnable on the triage seat's
own tool surface: MCP `search_issues` is semantic matching, so a literal
`Blocked-by:` needle returns 0 against a board where an open card's first body
line is exactly that string.

The half-state patrol already builds that reverse index for H14, over both
channels (body, plus comments for `pm:blocked` / `pm:blocking` cards whose body
is clean). So ④ is one lookup, not a new channel: this adds a rendered
inventory section beside the H17 trigger-file index — never a finding, never a
label — carrying the flagged cards and their waiters, a count pair, the H4
count as a declared NOT-MEASURED population, and the section's own population
scope, so a brief can cite both numbers or write NOT MEASURED honestly.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DAcomhvR9kKizeYgg89Vo8

Author: claude