Library functions need explicit session name argument — they no longer
have closure over $SESSION_NAME from the parent script.
- agent_kill_session: add $SESSION_NAME to all 11 call sites
- agent_inject_into_session: add $SESSION_NAME to all call sites in
phase-handler.sh and gardener-agent.sh
- agent_kill_session: guard against missing arg (defensive)
Both dev-agent.sh and gardener-agent.sh call these functions but they
were never implemented during the #158 extraction. Adds:
- create_agent_session(session, workdir) — tmux + claude + wait for ready
- inject_formula(session, text) — alias for agent_inject_into_session
