feat(sweep): close the aged FYI backlog without a human in the loop

A notice needs no answer, but it is still re-injected at every session
start until someone closes it by hand. Across enough repositories that
hand-closing IS the manual work the mailbox was meant to remove, and the
pending count -- the operator's only signal -- drowns in messages that
were never going to be acted on: 9 of 22 pending messages across 12
mailboxes were pure notices when this was written.

coord-sweep.sh closes exactly one mechanically decidable class:
reply-expected: no, older than a grace window (default 14 days). A
message that owes a reply is never touched, at any age, with any flag --
answering it would mean deciding something on the receiving repo's
behalf, which is the one thing this system exists to prevent.

Four properties are load-bearing, not incidental:

- Dry-run is the default, inverted from the rest of the engine. The
  others print or deliver; this one destroys pending state, so the safe
  direction has to be what you get by forgetting a flag.
- Closing goes through coord-done.sh --repo, never mv, so the archive
  layout and the _broadcast refusal stay in one place.
- Age is read from the filename prefix, never the file. An unreadable
  age is never treated as old: fail-safe, not fail-open.
- Every closure is logged with sender and subject. Directed messages
  have no seen-tracking, so the sweep cannot tell "seen and ignored"
  from "never delivered" -- a notice can be closed unread, and the log
  is the only thing standing between that and silent data loss.

The reply-expected read is bounded to the frontmatter block, matching
coord-count.sh: a body line claiming it at column 0 is untrusted
cross-repo input and must not close its own message.

No scheduler, no launchd unit, no skill front door -- the script does
nothing until invoked.

Selftest 159 -> 182. The log check caught a real defect during
development: the first implementation read from/subject AFTER
coord-done.sh had moved the file, logging empty values and quietly
defeating the only safeguard the design has.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uwcak9j4m9JijDKeFmptww

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.13.0",
+  "version": "0.14.0",
   "private": true,
   "type": "module",
   "engines": {