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)