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.stderr

@@ -0,0 +1,64 @@
+error: unused `Call` that must be used
+ --> tests/ui/must_use.rs:4:5
+  |
+4 |     call("p.m", ());
+  |     ^^^^^^^^^^^^^^^
+  |
+  = note: await the call to send it
+note: the lint level is defined here
+ --> tests/ui/must_use.rs:1:9
+  |
+1 | #![deny(unused_must_use)]
+  |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+  |
+4 |     let _ = call("p.m", ());
+  |     +++++++
+
+error: unused `TypedCall` that must be used
+ --> tests/ui/must_use.rs:5:5
+  |
+5 |     TypedCall::<()>::new(call("p.m", ()));
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: await the call to send it
+help: use `let _ = ...` to ignore the resulting value
+  |
+5 |     let _ = TypedCall::<()>::new(call("p.m", ()));
+  |     +++++++
+
+error: unused `Lock` that must be used
+ --> tests/ui/must_use.rs:6:5
+  |
+6 |     lock("/state/file");
+  |     ^^^^^^^^^^^^^^^^^^^
+  |
+  = note: await the lock to acquire it
+help: use `let _ = ...` to ignore the resulting value
+  |
+6 |     let _ = lock("/state/file");
+  |     +++++++
+
+error: unused `EndpointCall` that must be used
+ --> tests/ui/must_use.rs:7:5
+  |
+7 |     endpoint.call::<()>("m", ());
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: await the endpoint call to send it
+help: use `let _ = ...` to ignore the resulting value
+  |
+7 |     let _ = endpoint.call::<()>("m", ());
+  |     +++++++
+
+error: unused `Guard` that must be used
+  --> tests/ui/must_use.rs:10:24
+   |
+10 | fn dropped(g: Guard) { guard(g); }
+   |                        ^^^^^^^^
+   |
+   = note: bind the guard to hold the lock
+help: use `let _ = ...` to ignore the resulting value
+   |
+10 | fn dropped(g: Guard) { let _ = guard(g); }
+   |                        +++++++