feat(engine)!: counting is not delivering, and an empty inbox is not all clear

Reading IS delivery in this engine: coord-inbox.sh prints a broadcast and then
records it as seen. That made "what is pending elsewhere" unanswerable -- asking
the read path per repo would have consumed every repo's broadcast backlog as a
side effect, once, silently, and unrecoverably, since the seen set is delivery
history that retraction deliberately leaves alone.

coord-count.sh answers it by counting files and writing nothing: no seen set, no
.origin. It keys on MAILBOXES rather than repos -- it enumerates $COORD/* and
never scans a filesystem for checkouts -- so a repo without a mailbox is not
missing from the count, it is absent from the domain. Drained mailboxes are
omitted rather than reported as zero, because the question is "who is owed a
reply" and a list of zeroes answers a different one at every reader's expense.

The read path now closes with one aggregate line built from it. The case that
motivated this is the session whose own inbox is empty: it saw silence and
concluded "all clear" while mail sat unanswered everywhere else.

BREAKING (injection contract): the read path is no longer silent whenever THIS
repo has nothing pending. It is a silent no-op only when the whole mailbox is
empty. Coupling the line to having your own mail would have hidden it from its
only real audience. Three selftest assertions that used "no output at all" as a
proxy for "nothing was delivered" now assert the absence of the content itself,
which is what they always meant.

The line is an AGGREGATE of two integers, never a roster. A list of names would
reproduce other repos' situation inside this repo's injection -- the state
boundary the mailbox exists to respect -- and mailbox names are cross-repo
input. Two integers cannot carry anything that escapes the framing.

Its disclaimer is engine behavior, not politeness (Rule 7): the line lands
directly beneath "handle this inbox FIRST", and without it the numbers read as
an extension of that obligation and a session starts answering other repos'
mail. Pinned in selftest section 26 exactly as section 20 pins the priority
text. The hook's header drops "(unread messages)" for the same reason -- it
would now announce mail that does not exist.

Selftest 116 -> 136. Every new negative check is anchored to a positive
assertion in the same output, because a missing script makes "X is absent"
true by vacuity and would have gone green proving nothing.

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

CLAUDE.md

@@ -13,8 +13,17 @@ marketplace plugin. Three components, one boundary:
 - **Engine (`scripts/*.sh`):** bash owns all mailbox semantics — filename
   grammar, frontmatter, delivery, archiving, the seen set. `coord-send.sh`
   writes, `coord-inbox.sh` reads (formatted for context injection),
-  `coord-done.sh` archives. Everything is pinned by `coord-selftest.sh`
-  (116 checks, throwaway mailbox via `CLAUDE_COORD_DIR`).
+  `coord-done.sh` archives, `coord-count.sh` counts without delivering.
+  Everything is pinned by `coord-selftest.sh`
+  (136 checks, throwaway mailbox via `CLAUDE_COORD_DIR`).
+
+  **Reading is delivering — counting is not.** `coord-inbox.sh` records a
+  broadcast as seen once it has printed it, so it can never be used to survey
+  other repos: doing so would consume each one's backlog silently, and the seen
+  set is delivery history that retraction deliberately leaves alone.
+  `coord-count.sh` exists for every "what is pending" question and writes
+  nothing at all. Any future read-shaped feature belongs there, not in the
+  read path.
 - **Hook (`hooks/scripts/session-start.mjs`):** thin zero-dependency Node
   wrapper (marketplace convention: hooks are `.mjs`) that calls
   `coord-inbox.sh` and emits the `hookSpecificOutput.additionalContext`
@@ -54,7 +63,7 @@ halves together for exactly that reason.
 - Zero dependencies everywhere: bash + coreutils in the engine, `node:`
   builtins only in hook and tests.
 - TDD: no behavior change without a failing selftest check first.
-  `bash scripts/coord-selftest.sh` must exit 0 (116/116).
+  `bash scripts/coord-selftest.sh` must exit 0 (136/136).
 - English for all code, docs, and commit messages (public repo). Norwegian
   trigger aliases in the skill description are deliberate.
 - Conventional Commits: `type(scope): description`.