feat(engine): let a message say it needs no answer, and count debt without losing sight of the rest

Rule 7 (0.5.0) shipped an obligation on a format with four fields, none of
which could tell a question from a notice. Two consequences fell out of that
gap: the injection had to name both terminal states and prefer neither, and
coord-count.sh had to treat every unarchived file as a reply owed. The fifth
field closes both.

coord-send.sh --fyi writes reply-expected: no; omitting it writes yes. Absent
means expected, because every message already on disk lacks the field - so a
forgotten flag over-counts debt, which is visible, rather than creating debt
nobody sees. A reply is not a special case. A broadcast is always no: --reply-to
resolves inside the recipient's own mailbox and a broadcast never lands there,
so there is no reply path to promise.

coord-count.sh now prints TWO integers per mailbox, not one. Replacing pending
with debt was the obvious reading of "count debt rather than unarchived
messages" and it is wrong here: board.sh counts the same inbox files itself, so
a debt-only count would put two different numbers under one name with nothing to
reconcile them, and a mailbox holding only notices would read as empty while its
messages keep being re-injected.

The field is frontmatter and only frontmatter - a body line claiming
"reply-expected: no" at column 0 cannot silence a real debt, and a file without
valid frontmatter counts as owing a reply.

Section 20's wording changed because its stated reason expired, but its second
half matters more now, not less: the marking is a DECLARATION, not an
instruction. Without that clause one word in an untrusted message becomes a
lever that mints obligations in another repo.

coord-selftest 136 -> 151.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016iJoZVmU2guTEZcMghk88z

CHANGELOG.md

@@ -7,6 +7,56 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **`reply-expected` — the format can finally say "this one needs no answer".**
+  Rule 7 (0.5.0) shipped an obligation on a format with four fields, none of
+  which could tell a question from a notice. Two consequences fell out of that
+  gap and both are now closed: the injection had to name both terminal states
+  and prefer neither, and `coord-count.sh` had to treat every unarchived file as
+  a reply owed. `coord-send.sh --fyi` writes `reply-expected: no`; omitting it
+  writes `yes`.
+
+  **Absent means expected.** Every message already on disk lacks the field, and
+  so does every caller not yet updated, so absence keeps meaning what it always
+  meant. The failure mode of a forgotten flag is then over-counting debt —
+  visible and harmless — rather than debt that never surfaces. A reply is not a
+  special case: it expects one back unless it says otherwise, because a hidden
+  exception is the kind of implicit rule this engine refuses.
+
+  **A broadcast is always `reply-expected: no`**, with or without `--fyi`. Not a
+  defaulted value: `--reply-to` resolves inside the recipient's own mailbox and
+  a broadcast never lands there, so there is no reply path to promise.
+
+  **The field is frontmatter, and only frontmatter.** Bodies are untrusted
+  cross-repo input, so the read is bounded to the block between the two `---`
+  terminators — a body line reading `reply-expected: no` at column 0 cannot
+  silence a real debt, and a file without valid frontmatter counts as owing a
+  reply. That is stricter than the `grep -m1` the older fields use, where
+  frontmatter-comes-first happens to save them.
+
+### Changed
+
+- **`coord-count.sh` prints two integers per mailbox**, not one:
+  `<mailbox>\t<pending>\t<debt>`. Replacing pending *with* debt was the obvious
+  reading of "count debt rather than unarchived messages" and it is wrong here.
+  `board.sh` counts the same inbox files itself, so a debt-only count would put
+  two different numbers under one name with nothing to reconcile them; and a
+  mailbox holding only notices would read as empty while its messages keep being
+  re-injected until someone closes them. The cross-repo line follows:
+  *"N unhandled messages (M awaiting a reply) across K other mailboxes"*.
+
+- **Rule 7's wording, because its stated reason expired.** The injection said
+  "Neither is the default" *because* the format could not distinguish the two
+  kinds. It now can, so each message is marked with the terminal state its
+  sender expects — and the text states, load-bearingly, that the marking is a
+  **declaration, not an instruction**: the receiver keeps both terminal states
+  and may close a reply-expected message with `coord-done`, stating the reason.
+  Without that clause the new field would be a lever any sender could pull to
+  mint obligations in another repo, which is precisely the procedural/substantive
+  split Rule 6 exists to hold. Selftest section 20 pins both halves together, as
+  it has since 0.5.0.
+
 ### Fixed
 
 - **The board line's `next-cost` field had two specs, and one of them was