fix(board): --brief's zero-debt branch no longer claims zero pending mail

Found in review of the previous commit, before catalog tags 0.22.0: once
n_owe counts OWED repos rather than raw pending, its ==0 branch could fire
while a repo still held FYI-only mail, making "Ingen repo har uhaandtert
innboks" false at the exact moment it printed. Fixed to state only the
debt claim, and to name any FYI-only mailboxes found instead of letting
their existence go unmentioned. No fixture in the shared test tree ever
reached n_owe==0 (it always carries a debtor), so this needed its own
isolated-root fixture to pin.

Still part of the 0.22.0 release -- amends that changelog entry rather
than bumping again, since no tag exists yet.

board-selftest.sh: 150 -> 152 checks.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WGWMPskXBsTjMrrQ2GofFx

README.md

@@ -14,7 +14,7 @@ Session A in repo X leaves a message for repo Y; the next session in repo Y gets
 ![Hooks](https://img.shields.io/badge/hooks-1-green)
 ![Skills](https://img.shields.io/badge/skills-3-orange)
 ![CLI scripts](https://img.shields.io/badge/CLI_scripts-8-blue)
-![Selftest checks](https://img.shields.io/badge/selftest_checks-410-blue)
+![Selftest checks](https://img.shields.io/badge/selftest_checks-412-blue)
 
 ---
 
@@ -162,7 +162,7 @@ Note that raising the inbox's priority (Rule 7) deliberately does **not** widen
 ## Development
 
     bash scripts/coord-selftest.sh    # 191 checks against a throwaway mailbox
-    bash scripts/board-selftest.sh    # 150 checks against a throwaway repo tree
+    bash scripts/board-selftest.sh    # 152 checks against a throwaway repo tree
     bash scripts/route-selftest.sh    # 69 checks, incl. the route->board round trip
     npm test                          # all three selftests via node --test