fix(swarm-queue-client): export chain, and use it where anyhow used to

Review catch: `anyhow::Error`'s Display special-cases `f.alternate()` to
walk the source chain; thiserror's derive does not, so `{e:#}` and `{e}`
render identically for the new error type. Every call site that held an
`anyhow::Error`, formatted it with `{:#}`, and now holds this crate's
error kept compiling, kept looking right, and silently dropped the cause.

`chain()` was written for exactly this and then left private, applied only
to the auth callback I happened to be editing. Its own doc comment argues
that dropping the source chain is wrong, which made it the one thing in
the PR that should not have had a scope of one.

The controller's "swarm queue unreachable" warning is the site this fixes
here; the stacked PR fixes the two boot-warning banners, which matter more
still — one-shot, no retry, and they leak until restart.

swarm-controller/src/main.rs

@@ -318,7 +318,13 @@ async fn main() -> Result<()> {
                 )))
             }
             Err(e) => {
-                tracing::warn!(error = format!("{e:#}"), "swarm queue unreachable");
+                // `chain`, not `{:#}`: this is the queue client's own
+                // error type, and thiserror's Display ignores the
+                // alternate flag — the source would be dropped silently.
+                tracing::warn!(
+                    error = swarm_queue_client::chain(&e),
+                    "swarm queue unreachable"
+                );
                 None
             }
         },