fix(catalog): repo-mailbox stat line 398 -> 406 selftest checks

The catalog understated repo-mailbox's selftest count. Source of truth is the
plugin's own badge at the PINNED ref: `git show v0.20.3:README.md` line 17 says
`selftest_checks-406` (coord 191 + board 142 + route 73 = 406). The catalog said
398, measured at v0.20.2 before coord-selftest went 183 -> 191.

Flagged by repo-mailbox via coord; check-versions.mjs had it as the only ERROR
in the gate. That mattered beyond the stale number: release-plugin.mjs:200 calls
the gate through execFileSync, which throws on exit 1 -- AFTER writing
marketplace.json and README.md but BEFORE committing. Any `--write` run would
have landed a half-state, so this had to be green first.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XPz4FNKSn2QwoMhRFn4WNo

README.md

@@ -195,7 +195,7 @@ A local mailbox for coordination between Claude Code sessions in different repos
 - **`board`** — read-only cross-repo attention board: which repos have a live next step, which are blocked and on whom, which owe a reply, and what each costs to move
 - **`route`** — turns four scored traits of the next task into a model/effort row from a closed six-row rubric, with a pasteable startup command and a one-row-cheaper fallback
 
-1 hook · 3 skills · 8 CLI scripts · 398 selftest checks · [Full documentation →](https://git.fromaitochitta.com/open/repo-mailbox)
+1 hook · 3 skills · 8 CLI scripts · 406 selftest checks · [Full documentation →](https://git.fromaitochitta.com/open/repo-mailbox)
 
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