refactor: extract lib/agent-session.sh — reusable tmux + Claude agent runtime (#158)
Move generic agent infrastructure from dev/dev-agent.sh into lib/agent-session.sh:
- log, status, notify, notify_ctx, read_phase, wait_for_claude_ready,
inject_into_session, kill_tmux_session extracted verbatim
- create_agent_session(session_name, workdir) — new: tmux session creation
- inject_formula(session_name, formula_text, context) — new: prompt injection
- monitor_phase_loop(phase_file, idle_timeout, callback_fn) — new: phase loop
with session health check, crash recovery, and idle timeout detection
dev-agent.sh: sources the library, implements _on_phase_change() callback,
calls monitor_phase_loop(); idle-timeout and crash-recovery-failed cleanup
handled via _MONITOR_LOOP_EXIT signal variable. Behavior unchanged.
cbd8c81da8