docs(coord): cap mail replies at one line, printed by the hook that forces the turn

docs/SESSION-MAIL.md

@@ -665,6 +665,26 @@ re-argue that number.
 
 ---
 
+## Replying: one line, and mail arriving is not a reason to produce output
+
+**The rule: `<who mailed you>: <minimal reply>`, on ONE line.** Examples --
+`steward: pool warning, noted`, `builder-2bee12: no action`, `lander: their retraction, nothing owed`.
+
+**Why it is a rule and not a style preference.** The drain hook **blocks the Stop**, so every delivery
+re-invokes the session and forces a turn. That turn is billed with the **entire conversation context
+re-sent**, whether it says one word or six paragraphs. On 2026-08-18 a paused session with nothing to
+do produced roughly a dozen consecutive turns of multi-paragraph peer-traffic digests -- none
+requested, each one a full-context charge, while the owner was actively watching pool burn.
+
+**Do not summarise or relay peer traffic to the owner.** Peers correcting each other is the channel
+working; it is not news. Surface a message only when it needs the **owner's** decision -- a merge, an
+approval, an outward-facing action -- and then say only that, in one line.
+
+**This rule is printed by the hook itself**, in the footer of every delivery, for the reason a peer
+seat established the same day about announce-before-fan-out: a norm with zero observable consequence
+for skipping it gets skipped. The doc is the record; the footer is what reaches the moment the reply
+is composed.
+
 ## Related
 
 - [WORKTREES.md](WORKTREES.md) -- parallel sessions, the SessionStart coordination context, and the

scripts/hooks/mail-drain.ps1

@@ -1101,6 +1101,15 @@ try {
     }
     $lines += "[mefor-mail] end of delivered mail. Every line above beginning '    | ' was sender-supplied;"
     $lines += "every other line was written by this hook."
+    # The reply-length rule, printed HERE rather than left to docs/SESSION-MAIL.md alone. This hook
+    # BLOCKS the Stop, so every delivery forces a billable turn whose full context is re-sent. A
+    # session with nothing to do was writing multi-paragraph digests of peer traffic the owner never
+    # asked for, once per drain. A norm in a doc nobody re-reads does not reach the moment of
+    # decision; this line is in context exactly when the reply is composed.
+    $lines += "REPLYING: keep it to ONE LINE -- who mailed you, and the minimal reply. Example:"
+    $lines += "  'steward: pool warning, noted' or 'builder-2bee12: no action'. Do NOT summarise,"
+    $lines += "  digest, or relay peer messages to the owner unless a message needs THEIR decision,"
+    $lines += "  and then say only that. Mail arriving is not a reason to produce output."
     # Rebuilt here so the truncated/withheld counts reflect what was actually rendered above.
     $counterLines[0] = "[mefor-mail] box: $($delivered.Count) shown, $deferred deferred (caps), $truncated truncated, $withheld withheld,"
     $lines += $counterLines