fix: pass SESSION_NAME to all agent-session.sh function calls

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)

lib/agent-session.sh

@@ -60,5 +60,6 @@ inject_formula() {
 
 # Kill a tmux session gracefully (no-op if not found).
 agent_kill_session() {
-  tmux kill-session -t "$1" 2>/dev/null || true
+  local session="${1:-}"
+  [ -n "$session" ] && tmux kill-session -t "$session" 2>/dev/null || true
 }