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/agent-host-dev.sh

@@ -67,7 +67,8 @@ USER_FILE="${USER_FILE:-$MOSAIC_DATA_ROOT/user/USER.md}"
 PI="$REPO/node_modules/.bin/pi"
 [ -x "$PI" ] || fail 'run npm ci --ignore-scripts --no-audit --no-fund in the repository first'
 PIN="$(node -p "require('./package.json').dependencies['@earendil-works/pi-coding-agent']")"
-[ "$("$PI" --version)" = "$PIN" ] || fail "local Pi must match package.json ($PIN); run npm ci"
+PI_VERSION="$("$PI" --version)"
+[ "$PI_VERSION" = "$PIN" ] || fail "local Pi is $PI_VERSION, package.json pins $PIN; run npm ci in $REPO"
 command -v flock >/dev/null || fail 'flock is required'
 CONTEXT_FILES=("$CONSTITUTION" "$REPO/contracts/STANDARDS.md" "$SOUL" "$USER_FILE" "$REPO/AGENTS.md" "$REPO/agents/$AGENT/CONTEXT.md")
 for file in "${CONTEXT_FILES[@]}"; do