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/src/locks.rs

@@ -1,6 +1,7 @@
 //! Advisory lock acquisition and guard lifetime.
 use super::*;
 /// An advisory lock acquisition.
+#[must_use = "await the lock to acquire it"]
 pub struct Lock {
     path: String,
     wait: bool,
@@ -20,6 +21,7 @@ impl Lock {
     }
 }
 /// Releases its lock on drop, unless kept until process exit.
+#[must_use = "bind the guard to hold the lock"]
 pub struct Guard {
     path: Option<String>,
 }