feat(engine): retire a broadcast with coord-send --retract

Nothing could remove a message from _broadcast/inbox/. coord-done is
directed-only and never touches the broadcast queue, so the backlog could
only grow: every new repo received the entire standing history at its first
session, including announcements that had since become false.

--retract <filename> archives the message into _broadcast/archive/, so no
future repo is served it. Three deliberate limits, all pinned by tests:

- Un-send, not recall. Repos that already received it keep it;
  _broadcast/seen/ is delivery history and is left untouched.
- Only the sender may retract (from: must match the repo identity). --from
  overrides it, as everywhere else in the engine, which makes the check an
  accident guard rather than a security boundary.
- Nothing is deleted, mirroring coord-done. Retracting twice is a no-op.

The branch runs before every send-side validation and before the stdin body
read, since a retract carries no subject and no body.

Selftest 70 -> 82 (new section 19). Also fixes two README defects the
feature exposed: the install command still named coord@ after the v0.3.0
rename, and the docs advised pruning _broadcast/inbox/ by hand, which
contradicted the rule that the script owns mailbox files.

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "description": "Local mailbox for coordination between Claude Code sessions in different repositories. Directed messages and broadcasts as plain Markdown files on your own disk, injected as context at session start. Local, private, no network.",
   "author": {
     "name": "Kjell Tore Guttormsen"