Address review comments; fix placeholder binary reuse bug

release.go:
- Move manageBranchAndTag out of release-build (side-effect free)
- Fix KO_CONFIG_PATH to point to file, not dir
- Add --all to skopeo copy for full multi-arch manifest push

Dockerfile:
- Replace 'dummy' with 'placeholder' (woke linter)
- Delete target/ after placeholder pre-build to prevent cargo from
  reusing placeholder binary instead of compiling real sources
  (placeholder mtime > git-restored src/ mtime -> cargo skipped rebuild)

Assisted-by: 🤖 Claude Sonnet 4.6

Author: cardil

runner/Dockerfile

@@ -36,7 +36,7 @@ RUN mkdir src && echo 'fn main() {}' > src/main.rs && \
         cargo build --release --target aarch64-unknown-linux-gnu 2>/dev/null; true \
         ;; \
     esac && \
-    rm -rf src
+    rm -rf src target
 
 COPY . .