fix: paths-only skips branch validation, setup-plan preserves existing plan (#2672)

* fix: paths-only skips branch validation, setup-plan preserves existing plan (#2653)

- check-prerequisites.sh/ps1: move branch validation after --paths-only
  early exit so --paths-only returns paths without requiring a spec branch
- setup-plan.sh/ps1: skip template copy when plan.md already exists to
  prevent overwriting user-authored plans on reruns
- setup-plan.sh: send status messages to stderr in --json mode so stdout
  remains parseable JSON
- Add tests for both fixes (bash + PowerShell)

* fix: remove trailing whitespace in PowerShell scripts

* fix: route PS skip message to stderr in -Json mode, add PS JSON assertions

Address review: setup-plan.ps1 Write-Output polluted stdout in -Json
mode when plan.md already existed. Use [Console]::Error.WriteLine()
when -Json is set. Add json.loads + stderr assertions to the PS rerun
test to catch regressions.

* fix: use Test-Path -PathType Leaf for plan existence check

Bare Test-Path matches directories too, which would silently skip plan
creation if a directory existed at the plan.md path.

scripts/bash/check-prerequisites.sh

@@ -78,13 +78,12 @@ done
 SCRIPT_DIR="$(CDPATH="" cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/common.sh"
 
-# Get feature paths and validate branch
+# Get feature paths
 _paths_output=$(get_feature_paths) || { echo "ERROR: Failed to resolve feature paths" >&2; exit 1; }
 eval "$_paths_output"
 unset _paths_output
-check_feature_branch "$CURRENT_BRANCH" "$HAS_GIT" || exit 1
 
-# If paths-only mode, output paths and exit (support JSON + paths-only combined)
+# If paths-only mode, output paths and exit (no validation)
 if $PATHS_ONLY; then
     if $JSON_MODE; then
         # Minimal JSON paths payload (no validation performed)
@@ -112,6 +111,9 @@ if $PATHS_ONLY; then
     exit 0
 fi
 
+# Validate branch name
+check_feature_branch "$CURRENT_BRANCH" "$HAS_GIT" || exit 1
+
 # Validate required directories and files
 if [[ ! -d "$FEATURE_DIR" ]]; then
     echo "ERROR: Feature directory not found: $FEATURE_DIR" >&2