Add mosaic launch <seat> with seat registration for the control board (#1504)

New package packages/seat and wrapper scripts/mosaic. `launch <seat>` writes
<dataRoot>/seats/<layout>/<seat>/registration.json and then execs the seat's
launch.sh unchanged; `seat task <seat> <text>` edits the task only. The board
reads registrations, matches by sessions directory, and lets a registered
task, project or workspace override the derived value with a source tag.
The four repository launch scripts register themselves unless already
registered or run with --check. Fleet launchers untouched; one-liner on the
plan page.

Review found the record path keyed by seat name alone (repo and fleet
"darkwing" would collide); fixed by keying on layout. Also: the Pi pin
refusal now names installed and required versions.

Tests: seat 15, control-board 89, launch scripts 5, registry 69, config 24.

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

BUILD-LOG.md

@@ -2063,3 +2063,42 @@ a live pane, the rest from the log; every fleet seat reads `.mosaic` as
 its active project because `~/.mosaic` is a git checkout and the roster
 names no project. Missing launcher signals are listed in the plan page
 Step 3 log. Sonnet review: APPROVED, no findings.
+
+## 2026-09-12 — Seat registration: `mosaic launch <seat>` (#1504)
+
+Before: the board derived task, project and workspace from the session log
+and tmux, and had no way to know what a seat was actually told to do.
+Launch scripts left no record.
+
+After: `scripts/mosaic launch <seat|seat-dir> [--task ..] [-- args]`
+(package `packages/seat`) writes
+`<dataRoot>/seats/<layout>/<seat>/registration.json` (seat, project, task,
+workspace, tmux, harness, startedAt, pid, plus sessionsDir, seatDir,
+launchScript, layout, updatedAt, version) and then replaces itself with the
+seat's launch.sh via `process.execve`, so pid, terminal, exit code and the
+tmux foreground command are the script's own. `scripts/mosaic seat task
+<seat> <text>` edits the task only. The board reads every registration on
+each scan, matches by sessions directory, and lets a registered task,
+project or workspace override the derived value with `taskSource` /
+`activeProjectSource` / `workspaceSource` = `registration`; the page shows a
+"registered" tag and a Registered detail line. The four repository launch
+scripts re-enter through the launcher unless `MOSAIC_LAUNCH_REGISTERED` is
+set or `--check` is passed; fleet launchers are untouched, their one-line
+change is on the plan page for orch-01.
+
+Verification: seat suite 15/15, control-board 89/89 (was 79), launch-script
+suites 5/5 (each real launch.sh in a fixture that is its own data root),
+registry 69/69, config 24/24. Live: the real `agents/darkwing/launch.sh`
+through the launcher against a temporary data root, then a scratch board on
+7332 reading it; the darkwing row showed the registered task, both source
+tags and the Registered line, and a `seat task` change appeared with its
+update time.
+
+Corrections: sonnet review (first pass) found the record path keyed by seat
+name alone, which would have made the repo and fleet "darkwing" overwrite
+each other; fixed by keying on layout and seat, `seat task` refuses an
+ambiguous bare name unless `--layout` is given; second pass APPROVED. Found
+on the way: Jason's own launch refused on the Pi pin because
+`node_modules` still held 0.84.4 after commit 557aba0f moved the pin to
+0.85.1; `npm ci` fixed it and the refusal message now names both versions.
+The pin is canon and stays unless Jason decides otherwise.