ci: pilot-migrate clippy job to smithy self-hosted runners

Switches just the clippy job from ubuntu-latest to
[self-hosted, linux, x64, rust-cpu] — one of the three rust-cpu
runners on pulseengine-ci-01 (hetzner-private group).

Other jobs (fmt, test) stay on ubuntu-latest for now; once we have
a few green clippy runs and timing data, the rest can follow.

Why clippy first:
- meaningful compile work (good sccache test)
- bounded scope — failure doesn't block fmt or test
- no sudo, apt, or container needed
- spar already tracks nightly via dtolnay/rust-toolchain so the
  toolchain matches between hosted and self-hosted

If this PR's clippy job goes red on the self-hosted runner but
passes locally / on hosted, that's a smithy bug, not a code bug.

Author: avrabe

.github/workflows/ci.yml

@@ -31,7 +31,10 @@ jobs:
 
   clippy:
     name: Clippy
-    runs-on: ubuntu-latest
+    # smithy: pilot-migrating this single job to the pulseengine self-hosted
+    # fleet (hetzner-private runner group, rust-cpu class). Other jobs stay
+    # on ubuntu-latest until this one is proven over a few runs.
+    runs-on: [self-hosted, linux, x64, rust-cpu]
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@nightly