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.

scripts/uninstall.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+# Uninstall ctask from ~/.local/bin (or pass an alternative directory as $1).
+# Removes the binary and ctask-statusline.sh helper from the install directory.
+# Does NOT remove CTASK_ROOT, workspaces, or task data.
+
+set -eu
+
+INSTALL_DIR="${1:-$HOME/.local/bin}"
+
+printf 'ctask uninstall\n'
+printf '  Install dir: %s\n\n' "$INSTALL_DIR"
+
+if [ ! -d "$INSTALL_DIR" ]; then
+    printf '  Nothing to uninstall. Directory does not exist.\n'
+    exit 0
+fi
+
+# Only remove ctask-owned files
+removed=0
+for name in ctask ctask-statusline.sh; do
+    target="$INSTALL_DIR/$name"
+    if [ -f "$target" ]; then
+        rm -f "$target"
+        printf '  Removed %s\n' "$name"
+        removed=$((removed + 1))
+    fi
+done
+
+if [ "$removed" = "0" ]; then
+    printf '  No ctask files found in install directory.\n'
+fi
+
+printf '\n  PATH: not modified.\n'
+printf '  If you added %s to $PATH manually, remove the entry from your shell config.\n' "$INSTALL_DIR"
+
+printf '\nWorkspace data is untouched:\n'
+root="${CTASK_ROOT:-$HOME/ai-workspaces}"
+printf '  %s\n' "$root"
+
+printf '\nDone.\n'