fix: build static binary with --static-swift-stdlib

- Add --static-swift-stdlib flag to create self-contained binary
- Binary now includes Swift standard library statically linked
- Eliminates "libswiftCore.so not found" error on Ubuntu runners
- Allows sync workflow to run without Swift container

Inspired by: brightdigit/CelestraCloud workflow pattern

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: leogdion

.github/workflows/bushel-cloud-build.yml

@@ -40,8 +40,8 @@ jobs:
       - name: Build bushel-cloud executable
         id: build
         run: |
-          swift build -c release
-          BIN_PATH=$(swift build -c release --show-bin-path)
+          swift build -c release --static-swift-stdlib
+          BIN_PATH=$(swift build -c release --static-swift-stdlib --show-bin-path)
           echo "bin-path=$BIN_PATH" >> $GITHUB_OUTPUT
           echo "Binary location: $BIN_PATH/bushel-cloud"
           ls -lh "$BIN_PATH/bushel-cloud"