sdk, agent: review fixes (T18R)

- The serve loop ends the process after EOF or exit_after_response,
  also with background tasks or listeners.
- One schema generator per direction for the whole document, so types
  with the same name get distinct names.
- The agent gives the complete error (code, message, data) to the
  model, writes SKIP only after a successful request, caps tool output
  at 64 KiB, writes each history line in one write, and skips a damaged
  line.
- Endpoints skip blank and non-RPC lines. client_rs does not panic.
- #[must_use] on builders and the lock guard; #[schemars] attributes
  and duplicate hooks are compile errors; raw identifiers; -32000 for
  local SDK errors; x-sausage-params only for methods with params.

sdk/tests/ui/must_use.rs

@@ -0,0 +1,11 @@
+#![deny(unused_must_use)]
+use sausage_plugin::{call, lock, Endpoint, Guard, TypedCall};
+fn builders(endpoint: Endpoint) {
+    call("p.m", ());
+    TypedCall::<()>::new(call("p.m", ()));
+    lock("/state/file");
+    endpoint.call::<()>("m", ());
+}
+fn guard(g: Guard) -> Guard { g }
+fn dropped(g: Guard) { guard(g); }
+fn main() {}