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.

cmd/delete_test.go

@@ -30,7 +30,6 @@ func createTestWs(t *testing.T, root, category, dirName, status string) string {
 		Category:      category,
 		Mode:          "local",
 		Agent:         "claude",
-		WorkspacePath: dir,
 	}
 	workspace.WriteMeta(filepath.Join(dir, "task.yaml"), meta)