chore(release): bump repo-mailbox ref to v0.21.0

repo-mailbox removed route.sh's --opus-xhigh-failed flag and Fable rows
5-6 as a rubric outcome (operator policy decision) - a hand-written
override now, never produced by the calculator.

Known gate result: check-versions.mjs reports 1 expected ERROR for
repo-mailbox (catalog stat line still says 406 selftest checks, plugin's
own badge now says 402 - the release dropped three checks that tested a
command shape row_advisor() can no longer produce). repo-mailbox already
sent a coord-send correction superseding its earlier 406 notice with the
current 402; that stat-line prose is catalog's to update, not committed
here per the repo boundary. repo-standard's pre-existing WARN (v0.5.0 ref
vs 0.9.0 plugin.json, untagged) is unrelated and untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ap1WKHCDPcSfpjo4ds2dQX

.claude-plugin/marketplace.json

@@ -105,7 +105,7 @@
       "source": {
         "source": "url",
         "url": "https://git.fromaitochitta.com/open/repo-mailbox.git",
-        "ref": "v0.20.3"
+        "ref": "v0.21.0"
       },
       "description": "Local mailbox for coordination between Claude Code sessions in different repositories — directed messages and broadcasts injected as context at session start. Local, private, no network."
     },

README.md

@@ -185,7 +185,7 @@ End-to-end facilitator for prompting Claude Design (`claude.ai/design`). Walks y
 
 ---
 
-### [repo-mailbox](https://git.fromaitochitta.com/open/repo-mailbox) `v0.20.3`
+### [repo-mailbox](https://git.fromaitochitta.com/open/repo-mailbox) `v0.21.0`
 
 A local mailbox for coordination between Claude Code sessions in different repositories. Session A in repo X leaves a message for repo Y; the next session in repo Y gets it injected as context at startup. Local, private, no network, no SaaS.