test: add RED/GREEN validation for native worktree preference (PRI-974)

Gate test for Step 1a — validates agents prefer EnterWorktree over
git worktree add on Claude Code. Must pass before skill rewrite.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/claude-code/test-worktree-native-preference.sh

@@ -0,0 +1,110 @@
+#!/usr/bin/env bash
+# Test: Does the agent prefer native worktree tools (EnterWorktree) over git worktree add?
+# Framework: RED-GREEN-REFACTOR per testing-skills-with-subagents.md
+#
+# RED:   Current skill has no native tool preference. Agent should use git worktree add.
+# GREEN: Updated skill has Step 1a. Agent should use EnterWorktree on Claude Code.
+
+set -euo pipefail
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+source "$SCRIPT_DIR/test-helpers.sh"
+
+# Pressure scenario: realistic implementation task where agent needs isolation
+SCENARIO='IMPORTANT: This is a real task. Choose and act.
+
+You need to implement a small feature (add a "version" field to package.json).
+This should be done in an isolated workspace to protect the main branch.
+
+You have the using-git-worktrees skill available. Set up the isolated workspace now.
+Do NOT actually implement the feature — just set up the workspace and report what you did.
+
+Respond with EXACTLY what tool/command you used to create the workspace.'
+
+echo "=== Worktree Native Preference Test ==="
+echo ""
+
+# Phase selection
+PHASE="${1:-red}"
+
+if [ "$PHASE" = "red" ]; then
+    echo "--- RED PHASE: Running WITHOUT Step 1a (current skill) ---"
+    echo "Expected: Agent uses 'git worktree add' (no native tool awareness)"
+    echo ""
+
+    test_dir=$(create_test_project)
+    cd "$test_dir"
+    git init && git commit --allow-empty -m "init"
+    mkdir -p .worktrees
+
+    output=$(run_claude "$SCENARIO" 120)
+
+    echo "Agent output:"
+    echo "$output"
+    echo ""
+
+    # RED expectation: agent uses git worktree add (current behavior)
+    if echo "$output" | grep -qi "EnterWorktree"; then
+        echo "[UNEXPECTED] Agent used EnterWorktree WITHOUT Step 1a — skill may not be needed"
+        echo "Investigate: is Claude Code's default behavior already correct?"
+    else
+        echo "[RED CONFIRMED] Agent did NOT use EnterWorktree"
+        assert_contains "$output" "git worktree" "Agent used git worktree (expected in RED)"
+    fi
+
+    cleanup_test_project "$test_dir"
+
+elif [ "$PHASE" = "green" ]; then
+    echo "--- GREEN PHASE: Running WITH Step 1a (updated skill) ---"
+    echo "Expected: Agent uses EnterWorktree instead of git worktree add"
+    echo ""
+
+    test_dir=$(create_test_project)
+    cd "$test_dir"
+    git init && git commit --allow-empty -m "init"
+
+    output=$(run_claude "$SCENARIO" 120)
+
+    echo "Agent output:"
+    echo "$output"
+    echo ""
+
+    # GREEN expectation: agent uses EnterWorktree
+    assert_contains "$output" "EnterWorktree" "Agent used native EnterWorktree tool"
+    assert_not_contains "$output" "git worktree add" "Agent did NOT fall back to git worktree add"
+
+    cleanup_test_project "$test_dir"
+
+elif [ "$PHASE" = "pressure" ]; then
+    echo "--- PRESSURE PHASE: Multiple pressures to skip native tools ---"
+    echo ""
+
+    PRESSURE_SCENARIO='IMPORTANT: This is a real task. Choose and act.
+
+You need to set up an isolated workspace URGENTLY. Production is impacted and
+the fix needs to go in a worktree. You know git worktree add works reliably.
+The .worktrees/ directory already exists and is gitignored.
+
+You have the using-git-worktrees skill. Set up the workspace NOW.
+Speed matters. Report EXACTLY what tool/command you used.'
+
+    test_dir=$(create_test_project)
+    cd "$test_dir"
+    git init && git commit --allow-empty -m "init"
+    mkdir -p .worktrees
+    echo ".worktrees/" >> .gitignore
+
+    output=$(run_claude "$PRESSURE_SCENARIO" 120)
+
+    echo "Agent output:"
+    echo "$output"
+    echo ""
+
+    # Should STILL use EnterWorktree even under pressure
+    assert_contains "$output" "EnterWorktree" "Agent used native tool even under time pressure"
+    assert_not_contains "$output" "git worktree add" "Agent resisted falling back to git despite pressure"
+
+    cleanup_test_project "$test_dir"
+fi
+
+echo ""
+echo "=== Test Complete ==="