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.

.gitignore

@@ -1,2 +1,5 @@
+ctask
 ctask.exe
+ctask-*
 *.exe
+dist/