control board: reply from the board through agent-send.sh (#1505)

Piece 2 of the MVP (#1503). A one-line reply box and Send in the detail
of rows with a live registration; POST /api/reply runs
tools/tmux/agent-send.sh -s <session> -S <host>:control-board
[-L <socket>] -m <text> once for one seat and returns the exit code,
stdout and stderr. The page shows delivered or failed with the tool's
stderr; other rows say "reply needs a registered seat". No send-keys,
queue, retries, history or broadcast; packages/seat and agent-send.sh
untouched. Every message ends with a fixed trailer telling the seat to
answer in its own session (Jason's refinement after the first Gate C
exchange; the board has no pane). Board suite 98/98.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/control-board/README.md

@@ -64,8 +64,37 @@ Routes served:
   more than 4096 bytes): `{"project", "agent", "lastActivity", "seen"}`.
   `seen` defaults to `true`; pass `false` to unsee. Bad input, a missing
   header, or an oversized body gets a `400`.
+- `POST /api/reply` — send one line of text to one registered seat through
+  `tools/tmux/agent-send.sh` (#1505). Body: `{"agent": "<project>/<agent>",
+  "text"}`, JSON, at most 4096 bytes, text at most 2000 characters. The
+  server rescans, finds the row, and runs
+  `agent-send.sh -s <session> -S <host>:control-board [-L <socket>] -m <text>`
+  with the session and socket from the row's registration; `MOSAIC_TMUX_SOCKET`
+  is stripped from the tool's environment so the registration is the only
+  source. The text is sent as typed, followed on its own line by a fixed
+  trailer: `(control-board: answer in your own session as usual; the board
+  reads your transcript. Do not agent-send to control-board.)`. The board is
+  a sender without a pane: replies to it are read from the seat's
+  transcript, never sent, and `agent-send.sh -s control-board` refuses
+  because no such session exists (Jason's refinement after the first real
+  exchange, 2026-09-12). It answers `200` with `{delivered, exitCode, signal, stdout,
+  stderr, agent, session, socket, sentAt}` whenever the tool ran, delivered
+  or not; a non-zero exit is reported with the tool's stderr, never retried
+  and never routed around. Refusals before the tool runs: `400` bad body,
+  empty or blank text, text too long; `404` unknown row; `409` the row has
+  no registration, the registration is stale (pid gone), or it has no tmux
+  session; `500` the tool could not be started. One seat per request; no
+  queue, no history, no broadcast, no raw `send-keys`.
 - `GET /healthz`.
 
+On the page, the detail of a row with a live registration and a tmux
+session has a one-line reply box and a Send button; every other row's
+detail says "reply needs a registered seat". After Send the row shows
+`delivered <time> to tmux <session>` or `failed (exit N): <stderr>`. A
+draft that has not been sent, and the last receipt, survive the page's
+periodic refresh; a delivered reply clears the box and triggers a rescan
+so the seat's reaction shows without waiting for the next tick.
+
 ## Exit codes
 
 - `0` — scan completed and status files were written, or the server stopped cleanly.