feat(board)!: rank --plan on five ordered groups, planned above in-progress

Replaces the weighted score shipped in 0.19.0 with five lookups: chain-root
credit, unhandled inbox, planned, in-progress, undeclared status. Within a
group: that group's own quantity, then a Sonnet next-cost, then oldest plan.

The score's objection is accepted, not forgotten, and is written into board.sh
and CLAUDE.md so a later session reads it as decided rather than as an unfixed
defect: a group order cannot express "owes one message AND releases two others"
as one quantity. What the score could not do was hold still for the format's
second consumer - re-tuning one weight against another silently reorders a
parser in another repo, and no test here can catch that.

planned now ranks above in-progress, inverted by the same decision: converting a
decision into motion is the slow step; live work is already moving.

Debt stays uncapped and never excluded. One group below chain-root credit is not
the cap declined at 0.19.0 - the debtor keeps its tab, its most-owed-first
position, and its why=inbox:N. Pinned by a discriminating fixture the score
would fail: a root releasing one repo outranks a repo owing four.

board-selftest 134 -> 138. Suite 183 + 138 + 73 = 394.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y6ULuFCPMNYAPNN3pAjsXQ

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.19.0",
+  "version": "0.20.0",
   "private": true,
   "type": "module",
   "engines": {