feat(pm): H40 dangling-reference patrol over the live board's # references (#13964)

Sweeps the open board's bodies and already-cached comment threads for `#`
references into this repo, resolves each DISTINCT number once, and reports the
ones that fail to resolve with their referrers. Report-only.

⛔ closed != unreachable: only HTTP 404 is read as unresolvable; a closed card
resolves normally and is never a finding, and any other failure is UNJUDGED
rather than a vanished card.

Renders as a RESERVED section plus an unconditional summary clause, so the
anchor body's row trim cannot silently drop it.


Claude-Session: https://claude.ai/code/session_01Msg17tAHJ3jVTYFgHydCm2

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-sam