Check pi liveness per tmux pane and add "Seen" marks to the control board (#1503)

First step-3 refinement from Jason's daily use. Liveness now lists the
panes of the agent's tmux session and counts it alive only if a pane runs
pi, so killed pi sessions whose tmux session still exists show offline
instead of waiting. A "Seen" button on waiting and error rows stores the
row's lastActivity in <dataRoot>/board/seen.json (clicks only, never
rewritten by a scan, fail closed if corrupt) and drops the row from
"Waiting on you" until the agent writes anything newer; "Unsee" reverses
it. New POST /api/seen route: JSON only, 4 KB limit, 400 on bad input.

Tests: control-board 63/63 (30 new), registry 69/69. Review APPROVED;
receipt docs/plans/reviews/2026-09-12_control-board-step3-seen-marks.md.

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

docs/plans/CURRENT.md

@@ -17,6 +17,11 @@ are done in this checkout: `packages/control-board` scans every Pi agent
 Start it with `node packages/control-board/src/cli.mjs serve` and open
 http://127.0.0.1:7331/ (loopback only, no auth, no daemon; Ctrl-C stops it).
 
+First step-3 refinement landed 2026-09-12: liveness follows the tmux pane
+(killed pi sessions show offline) and a "Seen" button drops read rows out of
+"Waiting on you" until the agent writes again
+(`docs/plans/reviews/2026-09-12_control-board-step3-seen-marks.md`).
+
 Next action: step 3 of #1503 — daily use. Jason opens the page while he works
 and reports what is wrong, missing or confusing; each report becomes a small
 fix on this package with a test, committed and pushed to `refactor`. Close