feat(v0.5.1): Linux portability baseline

- justfile: add build-linux, build-windows, build-all (output to dist/)
- .gitignore: cover ctask, ctask-*, dist/
- scripts/install.sh + scripts/uninstall.sh: POSIX equivalents of .ps1
- remove WorkspacePath metadata field (no production readers; legacy
  task.yaml files continue to parse silently)

Linux smoke-test on WSL/container pending.
See audit-report.md and v0.5.1-spec.md.

justfile

@@ -10,6 +10,19 @@ default:
 build:
     go build -o ctask.exe .
 
+# Cross-compile a Linux amd64 binary into dist/
+build-linux:
+    mkdir -p dist
+    GOOS=linux GOARCH=amd64 go build -o dist/ctask-linux-amd64 ./
+
+# Cross-compile a Windows amd64 binary into dist/
+build-windows:
+    mkdir -p dist
+    GOOS=windows GOARCH=amd64 go build -o dist/ctask-windows-amd64.exe ./
+
+# Build both Windows and Linux artifacts into dist/
+build-all: build-windows build-linux
+
 # Run the full test suite
 test:
     go test ./... -v -count=1