Revert "docs(readme): drop the static selftest-checks badge"

This reverts commit 78bb997.

The badge isn't decorative: it's the input to the catalog's per-axis
stat-mirror-gate (check-versions.mjs:49-59), which cross-checks the
catalog's own stat line against this badge on every release. Dropping
it silently ungates that number -- exactly the defect class v0.20.2
was released to close, and the one check-versions.mjs cites by name
as "the standing proof that ungated numbers rot" (repo-mailbox's own
selftest count drifted to 251 while badge-less).

org-ops's census 06 (BADGE-STATIC-CLAIM) reads the badge as "a claim
dressed as evidence" because nothing visibly runs behind it. That's a
false-positive premise here: the count is verified, just off-badge --
by a script in another repo, at release time, not by a live CI run.
Answering the census with that mechanism instead of complying with
"link it or drop it".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015eUDLeEf8P4dithWDRhSqH

README.md

@@ -14,6 +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-402-blue)
 
 ---