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]>

packages/seat/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@mosaic/seat",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Seat launch and registration: `mosaic launch <seat>` runs a seat's launch script unchanged and leaves one registration record the control board reads.",
+  "license": "UNLICENSED",
+  "type": "module",
+  "engines": { "node": ">=24" },
+  "bin": { "mosaic": "src/cli.mjs" },
+  "exports": { ".": "./src/seat.mjs" },
+  "scripts": { "test": "node --test tests/" }
+}