Move the RPC wire schema into a Tauri-free crate (#553)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

crates/common/yaak-rpc-schema/README.md

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+# yaak-rpc-schema
+
+The wire schema for the app's RPC surface: every command name, its request
+payload, and its response type, declared once.
+
+Every host that serves the Yaak UI — the desktop app today, the browser bridge
+and anything after it — imports these types and implements the commands against
+them. That is what keeps a request's shape from drifting between hosts, and it
+is why the TypeScript bindings (`bindings/gen_rpc.ts`, exposed to the frontend
+as `@yaakapp-internal/rpc-schema`) are generated from one place.
+
+Nothing here depends on Tauri or on any host. Request structs are plain data,
+and so are the few response types declared here rather than in an engine crate.
+Command *bodies* live with the host that runs them.
+
+## Adding a command
+
+1. Add its request struct and an entry in `with_commands!` in `src/lib.rs`.
+2. Write the adapter in each host — the desktop's live in
+   `crates-tauri/yaak-app-client/src/rpc_ext.rs`. A host that does not support
+   the command still has to say so; a missing adapter fails to compile.
+3. Regenerate the bindings: `cargo test -p yaak-rpc-schema` writes
+   `bindings/gen_rpc.ts`, which is committed.
+
+## How hosts consume the list
+
+`with_commands!` takes the name of a `macro_rules!` macro and calls it with the
+full `name(Req) -> Res` list. Each host writes a small macro that receives that
+list and builds its router:
+
+```rust
+macro_rules! register_commands {
+    ( $( $name:ident ( $req:ty ) -> $res:ty ),* $(,)? ) => {
+        pub fn build_router() -> RpcRouter<MyCtx> {
+            let mut router = RpcRouter::new();
+            $( router.register(stringify!($name), rpc_handler_async!($name)); )*
+            router
+        }
+    };
+}
+yaak_rpc_schema::with_commands!(register_commands);
+```
+
+The schema decides *what* commands exist; the host decides *how* each one runs.