url deep-link: game id in #/game/<id>, multi-tab + bad-link fallback (v0.3.0)

AGENTS.md

@@ -62,7 +62,7 @@ history/
 │   ├── db.ts               # SQLite open/save/load
 │   └── *.test.ts           # unit tests (vitest)
 └── src/
-    ├── App.tsx             # screen router: home → pick → game; owns game/regions/advisors/busy state
+    ├── App.tsx             # screen router (home → pick → game) + manual hash router (#/game/<id>); owns game/regions/advisors/busy
     ├── main.tsx            # React root + ThemeProvider(dark)
     ├── index.css           # Tailwind v4 + dark "command-center" theme
     ├── lib/
@@ -104,6 +104,13 @@ history/
 - **No hardcoded origins.** The client uses same-origin relative paths (`/api/...`, `/sse/...`).
   In dev, Vite proxies them to :4321; in prod/Traefik they're the same origin. So the app works
   unchanged behind the proxy — never bake in `http://host:port`.
+- **Hash-based routing (client-only).** The active game's id lives in the URL fragment: home is
+  `#/` (a bare page with no hash is also home) and an active game is `#/game/<id>`. This is a manual
+  hash sync in `App.tsx` — no react-router, no server route (every non-API GET just serves
+  `index.html`), so it is Traefik/static-safe. A deep link opens that exact game and two tabs can
+  run two different games (each has its own React state + `useSse(game.id)`). The URL hash takes
+  precedence over the localStorage active-game id, which remains the fallback for a bare `/` load;
+  a bad/missing id clears the hash to `#/` and falls back to home.
 - **Batched round model.** Actions and diplomatic messages are queued in `game.pending` via
   `POST /action` and `POST /diplomacy` (fast, no LLM call, no turn advance). A single
   `POST /jump` ("Jump forward") commits the whole pending batch in one LLM turn: `engine.runTurn`