Initial StartOS package for Station Master (v0.5.1)

Built from source via a git submodule pinned to a tag, not a published image
— the Dockerfile, main.ts, and manifest should never need to change for an
ordinary version bump, only the submodule pin (see UPDATING.md). One volume,
one interface serving the browser client + lobby/intent API + SSE stream
from a single origin, no dependencies. The server-wide join secret (D14) is
seeded on install, exposed via the Get Join Secret action, and blocks start
behind a critical task until retrieved — the same first-set/rotation pattern
as actual-budget-startos's admin password.

Verified on phoenix.local: installs, the critical task correctly blocks an
ordinary start, force-starting confirms the daemon binds its port and serves
the client, and store.json correctly holds the install-seeded join secret.
Not verified: the Get Join Secret action's execution end-to-end — start-cli's
`package action run` fails with a client-side deserialization error that
reproduces identically against actual-budget's already-shipped equivalent
action, so this looks like a start-cli issue rather than a defect here.

icon.svg is still the scaffold's hello-world placeholder — no real Station
Master icon exists yet to ship in its place.

.github/workflows/build.yml

@@ -0,0 +1,17 @@
+name: Build
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths-ignore: ['*.md']
+    branches: ['master']
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    if: github.event.pull_request.draft == false
+    uses: Start9Labs/start-technologies/.github/workflows/build.yml@master
+    # No DEV_KEY — a PR build doesn't publish, so it doesn't need the signing key.