test: add environment detection tests for Codex App compat (PRI-823)

Tests git-dir vs git-common-dir comparison in normal repo, linked
worktree, detached HEAD, and cleanup guard scenarios.

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

tests/codex-app-compat/test-environment-detection.sh

@@ -0,0 +1,94 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Test environment detection logic from PRI-823
+# Tests the git-dir vs git-common-dir comparison used by
+# using-git-worktrees Step 0 and finishing-a-development-branch Step 1.5
+
+PASS=0
+FAIL=0
+TEMP_DIR=$(mktemp -d)
+trap "rm -rf $TEMP_DIR" EXIT
+
+log_pass() { echo "  PASS: $1"; PASS=$((PASS + 1)); }
+log_fail() { echo "  FAIL: $1"; FAIL=$((FAIL + 1)); }
+
+# Helper: run detection and return "linked" or "normal"
+detect_worktree() {
+  local git_dir git_common
+  git_dir=$(cd "$(git rev-parse --git-dir)" 2>/dev/null && pwd -P)
+  git_common=$(cd "$(git rev-parse --git-common-dir)" 2>/dev/null && pwd -P)
+  if [ "$git_dir" != "$git_common" ]; then
+    echo "linked"
+  else
+    echo "normal"
+  fi
+}
+
+echo "=== Test 1: Normal repo detection ==="
+cd "$TEMP_DIR"
+git init test-repo > /dev/null 2>&1
+cd test-repo
+git commit --allow-empty -m "init" > /dev/null 2>&1
+result=$(detect_worktree)
+if [ "$result" = "normal" ]; then
+  log_pass "Normal repo detected as normal"
+else
+  log_fail "Normal repo detected as '$result' (expected 'normal')"
+fi
+
+echo "=== Test 2: Linked worktree detection ==="
+git worktree add "$TEMP_DIR/test-wt" -b test-branch > /dev/null 2>&1
+cd "$TEMP_DIR/test-wt"
+result=$(detect_worktree)
+if [ "$result" = "linked" ]; then
+  log_pass "Linked worktree detected as linked"
+else
+  log_fail "Linked worktree detected as '$result' (expected 'linked')"
+fi
+
+echo "=== Test 3: Detached HEAD detection ==="
+git checkout --detach HEAD > /dev/null 2>&1
+branch=$(git branch --show-current)
+if [ -z "$branch" ]; then
+  log_pass "Detached HEAD: branch is empty"
+else
+  log_fail "Detached HEAD: branch is '$branch' (expected empty)"
+fi
+
+echo "=== Test 4: Linked worktree + detached HEAD (Codex App simulation) ==="
+result=$(detect_worktree)
+branch=$(git branch --show-current)
+if [ "$result" = "linked" ] && [ -z "$branch" ]; then
+  log_pass "Codex App simulation: linked + detached HEAD"
+else
+  log_fail "Codex App simulation: result='$result', branch='$branch'"
+fi
+
+echo "=== Test 5: Cleanup guard — linked worktree should NOT remove ==="
+cd "$TEMP_DIR/test-wt"
+result=$(detect_worktree)
+if [ "$result" = "linked" ]; then
+  log_pass "Cleanup guard: linked worktree correctly detected (would skip removal)"
+else
+  log_fail "Cleanup guard: expected 'linked', got '$result'"
+fi
+
+echo "=== Test 6: Cleanup guard — main repo SHOULD remove ==="
+cd "$TEMP_DIR/test-repo"
+result=$(detect_worktree)
+if [ "$result" = "normal" ]; then
+  log_pass "Cleanup guard: main repo correctly detected (would proceed with removal)"
+else
+  log_fail "Cleanup guard: expected 'normal', got '$result'"
+fi
+
+# Cleanup worktree before temp dir removal
+cd "$TEMP_DIR/test-repo"
+git worktree remove "$TEMP_DIR/test-wt" > /dev/null 2>&1 || true
+
+echo ""
+echo "=== Results: $PASS passed, $FAIL failed ==="
+if [ "$FAIL" -gt 0 ]; then
+  exit 1
+fi