fix: harden bash scripts against shell injection and improve robustness (#1809)

- Replace eval of unquoted get_feature_paths output with safe pattern:
  capture into variable, check return code, then eval quoted result
- Use printf '%q' in get_feature_paths to safely emit shell assignments,
  preventing injection via paths containing quotes or metacharacters
- Add json_escape() helper for printf JSON fallback paths, handling
  backslash, double-quote, and control characters when jq is unavailable
- Use jq -cn for safe JSON construction with proper escaping when
  available, with printf + json_escape() fallback
- Replace declare -A (bash 4+) with indexed array for bash 3.2
  compatibility (macOS default)
- Use inline command -v jq check in create-new-feature.sh since it
  does not source common.sh
- Guard trap cleanup against re-entrant invocation by disarming traps
  at entry
- Use printf '%q' for shell-escaped branch names in user-facing output
- Return failure instead of silently returning wrong path on ambiguous
  spec directory matches
- Deduplicate agent file updates via realpath to prevent multiple writes
  to the same file (e.g. AGENTS.md aliased by multiple variables)

scripts/bash/setup-plan.sh

@@ -28,7 +28,9 @@ SCRIPT_DIR="$(CDPATH="" cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/common.sh"
 
 # Get all paths and variables from common functions
-eval $(get_feature_paths)
+_paths_output=$(get_feature_paths) || { echo "ERROR: Failed to resolve feature paths" >&2; exit 1; }
+eval "$_paths_output"
+unset _paths_output
 
 # Check if we're on a proper feature branch (only for git repos)
 check_feature_branch "$CURRENT_BRANCH" "$HAS_GIT" || exit 1
@@ -49,8 +51,18 @@ fi
 
 # Output results
 if $JSON_MODE; then
-    printf '{"FEATURE_SPEC":"%s","IMPL_PLAN":"%s","SPECS_DIR":"%s","BRANCH":"%s","HAS_GIT":"%s"}\n' \
-        "$FEATURE_SPEC" "$IMPL_PLAN" "$FEATURE_DIR" "$CURRENT_BRANCH" "$HAS_GIT"
+    if has_jq; then
+        jq -cn \
+            --arg feature_spec "$FEATURE_SPEC" \
+            --arg impl_plan "$IMPL_PLAN" \
+            --arg specs_dir "$FEATURE_DIR" \
+            --arg branch "$CURRENT_BRANCH" \
+            --arg has_git "$HAS_GIT" \
+            '{FEATURE_SPEC:$feature_spec,IMPL_PLAN:$impl_plan,SPECS_DIR:$specs_dir,BRANCH:$branch,HAS_GIT:$has_git}'
+    else
+        printf '{"FEATURE_SPEC":"%s","IMPL_PLAN":"%s","SPECS_DIR":"%s","BRANCH":"%s","HAS_GIT":"%s"}\n' \
+            "$(json_escape "$FEATURE_SPEC")" "$(json_escape "$IMPL_PLAN")" "$(json_escape "$FEATURE_DIR")" "$(json_escape "$CURRENT_BRANCH")" "$(json_escape "$HAS_GIT")"
+    fi
 else
     echo "FEATURE_SPEC: $FEATURE_SPEC"
     echo "IMPL_PLAN: $IMPL_PLAN"