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]>

scripts/mosaic

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# `mosaic launch <seat>` and `mosaic seat task <seat> <text>`: seat launch
+# with registration for the control board. See packages/seat/README.md.
+# Not the npm-global `mosaic` CLI from the estate tooling; this one is
+# repository-local and only reachable as scripts/mosaic.
+set -euo pipefail
+REPO="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+exec node "$REPO/packages/seat/src/cli.mjs" "$@"