feat(engine)!: the inbox is a priority, not a suggestion

Through 0.4.0 the injection block told every repo to "consider
replying/resolving where it fits in this session". That sentence was the
whole problem: the injection text is the only place a repo is ever told
what to do with a message, so the wording IS the protocol -- and it
granted permission to defer. Messages sat unanswered for weeks while each
session did its own work first. Nothing was broken; the protocol was
asking for exactly what it got.

The block now states an ordering and a completion obligation: handle the
inbox before the task the session came to do, and drive every directed
message to a terminal state before the session ends (--reply-to or
coord-done). Neither terminal state is the default -- the format has no
reply-expected field, so mandating only the reply would manufacture
traffic for messages that merely inform. Leaving one pending stays
allowed but must be stated to the operator with a reason.

Raising priority deliberately does not widen the trust boundary. The
obligation is procedural, never substantive: responding is mandatory,
complying with what a message asks is not. Untrusted cross-repo content
still cannot direct the reader; it merely can no longer be ignored. The
injection states both halves and selftest section 20 pins them together,
so a future reword cannot keep the priority and quietly drop the
distinction -- that combination would turn prioritization into an
injection surface.

Selftest 82 -> 93.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U6EixQo6hpoRCVtiAXdnFs

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.4.0",
+  "version": "0.5.0",
   "private": true,
   "type": "module",
   "engines": {