chore: retire the maonakamoto handle

The account was renamed to `catomean` and the repos then moved to the
`bitbaum` org, so `maonakamoto` now names nothing: github.com/maonakamoto
and maonakamoto.github.io both return 404, and @maonakamoto no longer
resolves as a user.

Three different targets, because these are not the same reference:
  repo links  -> bitbaum/<repo>   (the repos live in the org)
  the person  -> catomean        (profile links, @mentions, CODEOWNERS)
  pages site  -> bitbaum.github.io

Deliberately NOT changed:
  .mailmap keeps the old noreply address as an ALIAS. It is what maps
  historical commits onto the current identity; deleting it would
  silently unmap them. Only the canonical side moved to catomean.
  Files that explain the outage by naming the old account keep it,
  since the name is the content.

Files: package.json

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UvjGNAS9CMfEGNW26tUR4P

Author: catomean

package.json

@@ -4,13 +4,13 @@
   "description": "Headless multi-participant message threads. Permission is participation, unread is per-person, and an AI can be a participant subject to the same visibility rules as everyone else.",
   "license": "MIT",
   "author": "Mao Nakamoto",
-  "homepage": "https://github.com/maonakamoto/threadkit#readme",
+  "homepage": "https://github.com/bitbaum/threadkit#readme",
   "repository": {
     "type": "git",
-    "url": "git+https://github.com/maonakamoto/threadkit.git"
+    "url": "git+https://github.com/bitbaum/threadkit.git"
   },
   "bugs": {
-    "url": "https://github.com/maonakamoto/threadkit/issues"
+    "url": "https://github.com/bitbaum/threadkit/issues"
   },
   "keywords": [
     "messaging",