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]>

docs/plans/CURRENT.md

@@ -32,13 +32,27 @@ Package: `packages/seat`. Fleet seats stay on their own launchers (Jason's
 ruling, 2026-09-12); only `agents/` seats register. A record whose pid is
 gone is stale and does not override the derived values.
 
-Next action: step 3 of #1503 continues — daily use with #1504 landed. Jason
-launches his seats through the new path (or the launch scripts, which now
-register), sets tasks with `seat task`, and reports what is wrong, missing
-or confusing; each report becomes a small fix with a test, committed and
-pushed to `refactor`. The sequence Jason set stays: seat registration (done),
-then reply-from-board, then the WebUI on Dewey's Console design absorbs the
-board as its first screen. Close #1503 when Jason says the page answers "who
+Gate B passed 2026-09-12 (seat registration shown and used on the board).
+
+Piece 2, reply-from-board (#1505; brief in the plan page, section "Piece
+2: reply-from-board", approved by Jason 2026-09-12), is built and pushed
+2026-09-12: a text box and Send in the detail of registered, live rows;
+`POST /api/reply` runs `tools/tmux/agent-send.sh -s <session> -S
+"<host>:control-board" [-L <socket>] -m <text>` and returns the exit code,
+stdout and stderr; the page shows `delivered` or `failed` with the stderr.
+No send-keys, queue, history or broadcast; no change to `packages/seat` or
+`agent-send.sh`. Board suite 98/98.
+
+Gate C's first exchange happened 2026-09-12 ("pizza?" to darkwing:
+delivered, read back within one scan; the seat's send-back to
+`control-board` was refused as expected, so every board message now ends
+with a fixed trailer telling the seat to answer in its own session).
+
+Next action: Jason confirms Gate C on the trailer build (restart the board
+on 7331, answer a `waiting` seat, the row turns `working` with the reply
+as its newest user message and the seat answers in its own session).
+Record the pass or the failure in the plan page and here. Then the WebUI on Dewey's Console design absorbs
+the board as its first screen. Close #1503 when Jason says the page answers "who
 is waiting on me" without him opening a terminal. Out of scope until he
 asks: auth or provider registry, roster schema changes, hooks/plugins, comms,
 memory, multiple sessions per seat, stopping seats, new root files. The