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)
kill_tmux_session → agent_kill_session
inject_into_session → agent_inject_into_session
wait_for_claude_ready → agent_wait_for_claude_ready
Also restore status() function lost during #160 refactor.
Fixes dev-agent and gardener-agent crash on startup:
line 149: status: command not found
line 280: kill_tmux_session: command not found
