ci: build/test tests/spm instead of plugin-only root

The repo root is a plugin-only package with no buildable target, so
`swift build` there fails ("does not contain a buildable target").
Building the tests/spm harness compiles the a11y-scan command plugin via
the path dependency plus the sample sources, so use that as the build step.

Verified locally on Swift 6.2: tests/spm `swift build` compiles the plugin,
and `swift test` passes with the e2e scan test skipping when RUN_A11Y_SCAN
is unset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Crash0v3rrid3

.github/workflows/spm-smoke-test.yml

@@ -44,7 +44,11 @@ jobs:
       - name: Swift toolchain
         run: swift --version
 
-      - name: Build plugin package
+      # The repo root is a plugin-only package (no buildable target), so it is
+      # not built directly. Building the tests/spm harness compiles both the
+      # a11y-scan command plugin (via the path dependency) and the sample sources.
+      - name: Build harness (compiles the a11y-scan plugin)
+        working-directory: tests/spm
         run: swift build
 
       - name: End-to-end scan smoke (tests/spm)