[codex] Fix macOS Bash 3 compatibility

[samleeney]

Summary

Fixes #32.

• Adds a shared Bash 4+ requirement helper that re-execs tmux-agent-status entrypoints with a modern Bash when macOS starts them under /bin/bash 3.2.
• Detects common Homebrew, MacPorts, and Fink Bash paths, with TMUX_AGENT_STATUS_BASH as an explicit override.
• Wires the helper into session-status.sh, which is sourced before the reported associative-array failures.
• Documents the macOS brew install bash prerequisite and adds a focused helper test.

Why

macOS ships Bash 3.2, which does not support declare -A or local -A. The sidebar/status scripts use associative arrays in shared collection code, so they need to run under Bash 4 or newer instead of failing when launched from tmux on macOS.

Validation

• for f in $(rg --files scripts hooks tests | sort); do bash -n "$f"; done
• for t in tests/*.sh; do bash "$t"; done
• TMUX_AGENT_STATUS_BASH="$(command -v bash)" bash tests/require-bash4-helper.sh
• ./scripts/sidebar-signal.sh refresh; echo exit:$?