chore(release): bump to 0.31.0

Two commits landed after v0.30.0 and neither reaches anyone: every entry
point on this machine runs the version-pinned plugin cache, not the tree.
Measured 2026-08-29 by .claude before this order was written - cache
board.sh 1626 lines against the repo's 1739, md5 differing - so the board
skill was still running pre-4369499 code. A commit is not a delivery.

Minor rather than patch: ee5a86a adds a reading the table did not have
(ORDRE and FLY carry the age of the oldest order), while 4369499 repairs
a consumer that discarded a signal the engine already sent. No new
key=value key, no new exit code, no new env var, no sort-axis change.

Version bumped in all seven files that carry it: .claude-plugin/plugin.json,
package.json, the README badge and the four SKILL.md frontmatters. The
0.30.0 strings remaining in CLAUDE.md, board.sh and board-selftest.sh are
historical prose about what was measured on that release and are correct
as they stand - a blanket sed would have falsified them.

CHANGELOG 0.31.0 written for a reader of the board rather than a reader
of the diff: what the columns now say, and what --brief no longer claims.

Suites under real /bin/bash 3.2 after the bump: coord 242/242, board
325/325, route 69/69, orders 110/110, guard 54/54. npm test 11/11.

No tag and no catalog change, as ordered - release-plugin.mjs lives in
catalog and mints the tag and the ref atomically; a hand-made tag here is
exactly the drift that script exists to prevent.

Order: 20260829T071929Z-786087280-from-.claude

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.30.0",
+  "version": "0.31.0",
   "private": true,
   "type": "module",
   "engines": {