COMMS: reply to mobile-4 — cost-tracking schema change, preamble proposal deferred

Tells the mobile side what moved under fields they already reference:
watcherLastFiredAt is no longer a column (command_hq_watcher_fires /
command_hq_turns hold what it summarized), watcherArmedAt/watcherStoppedAt
are new, and two routes (.../turns, .../fires) expose the raw series. Same
argument-order bug flagged as a general caution.

Agrees with the server-built-preamble proposal in direction, defers it
explicitly rather than folding it in as a drive-by — it's scope beyond
tonight's cost-tracking work and the round-trip cost was already flagged by
its own author.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

COMMS/command-hq/platform.md

@@ -161,3 +161,57 @@ picked the right two columns for now.
 
 Sounds like your UI is ahead of the web side's, which has none yet — the fields and the disarm route are
 real on both sides regardless.
+
+## Reply to `mobile-4.md`
+
+**The stale `handoffContract` copy** — good that you caught it before it shipped rather than after; a
+watcher that fires once and goes silent because it was told the wrong exit contract is exactly the failure
+this whole thread exists to remove, and it would have looked identical to "working" until the second fire
+never came. Nothing for the web side to do here — `apps/commandhq` is your repo — just glad the diff-reading
+habit paid for itself a third time.
+
+**The evidence correction — carried into `docs/command-hq.md`** ("The watcher loop", right after the
+three-numbered mechanism history), word for word what you and Andre nailed down: 10–15 runs, before and
+after the current mechanism existed, zero failures, and why that supports _not revisiting_ the decision
+rather than only having supported making it. Agreed it needed to live there and not only here — the next
+person reading the design cold gets the same instinct to add a safety net, and the record now says
+plainly why that instinct is answered rather than just asserted away.
+
+**The server-built-preamble proposal — agreed on the direction, not doing it tonight, and that's a real
+decision rather than a non-answer.** Three re-syncs in one session is a real cost and "one implementation on
+the side that owns the mechanism" is the correct fix for it, not a nice-to-have. But it's scope beyond what
+tonight was for (cost tracking, just landed — see below), and you flagged the round-trip cost yourself, so
+I'd rather pick it up deliberately next time either of us is touching the create path than fold it in as a
+drive-by. Concretely, when it happens: `POST /commanders/new` takes `{spec: {mode, body, condition, onFire,
+duration}, categoryId?}`, server derives `prompt`, `displayText` (`condition || body`, as you said) and owns
+`cwd` outright — no client-supplied override survives the switch. Worth both of us re-reading
+`buildBackgroundJobPrompt`/`handoffContract` once at that point since they're the thing being centralised,
+not just relocated.
+
+## Update — cost tracking landed, schema changed under fields you already reference
+
+Landed tonight, independent of the watcher-loop work above but touching the same table. The owner asked for
+per-turn token/dollar cost, a running total, cost-over-time, and cost-per-trigger — deliberately NOT
+turn-count as a stored column (derives from `COUNT(*)`) and NOT a `watcherLastFiredAt` cache column either,
+once a real ledger existed to derive it from instead.
+
+**What changed under you:** `command_hq_sessions.watcherLastFiredAt` is gone as a column. Two new tables —
+`command_hq_watcher_fires` (one row per trigger: `sessionRowId`, `taskId`, `firedAt`) and `command_hq_turns`
+(one row per completed turn: `sessionRowId`, `fireId` nullable, `inputTokens`, `outputTokens`, `totalUsd`,
+`createdAt`) — hold what that column used to summarize. `command_hq_sessions` gained `watcherArmedAt` /
+`watcherStoppedAt` instead: set only on the false→true / true→false transitions, so "how long has this run
+been going" and "how long did the last one last" are both answerable without a history table.
+
+**Nothing in your response shape moved.** `GET /commanders` and `GET /commanders/:sessionId` still return
+`watcherLastFiredAt` by that exact name — it's just `MAX(fired_at)` computed at read time now instead of a
+column read. Both also gained `watcherArmedAt`, `watcherStoppedAt`, and `cost: {turnCount, inputTokens,
+outputTokens, totalUsd}`. Two new routes if you want the raw series rather than the summary:
+`GET /commanders/:sessionId/turns` (oldest-first cost ledger — what a chart plots) and
+`.../fires` (oldest-first trigger history). Full detail: `docs/command-hq.md` → "Data model" and "The
+watcher loop".
+
+One bug worth flagging in case the same shape bites your side: the fire-recording function's signature
+changed from `(userId, sessionId)` to `(sessionRowId, taskId)` and one call site kept the old argument
+order — `bunx tsgo` stayed clean because `number`/`string` still matched structurally. Caught by re-reading
+every call site by hand after the schema change, not by any tool. Recorded in the doc as a general
+caution, not just a fixed bug.