artifacts: archive non-report dirs at their own paths, list them on the artifact page

Every artifactDir landed below reports/ — the legacy
archived_artefact_dir_path layout — which mislabeled non-report outputs
(reports/werkdock/dist/werkdock) AND hid them: the artifact page's
report index only scans reports/ for HTML pages, so a built binary was
stored but never shown.

Now build/reports keeps archiving as reports/ (the browsable anchor and
every existing link), every other directory archives at its
workspace-relative path, and the artifact page gains a plain-files list
for everything outside reports/ (log files stay in their own section;
capped at 200 entries). Existing stored artifacts keep their old layout
and remain served; only new builds use the new one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/configuration.md

@@ -201,7 +201,9 @@ builds:
     cleanCommand: rm -rf build
     # shell command for each build
     buildCommand: ./gradlew --console=plain --no-daemon test
-    # directories copied as build artifacts
+    # directories copied as build artifacts; each is archived at its own
+    # workspace-relative path, except build/reports, which archives as reports/
+    # and is browsed by the artifact page's report index
     artifactDirs:
       - build/reports
       - build/doc