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)

dev/dev-agent.sh

@@ -100,7 +100,7 @@ CLAIMED=false
 cleanup() {
   rm -f "$LOCKFILE" "$STATUSFILE"
   # Kill any live session so Claude doesn't run without an orchestrator attached
-  agent_kill_session
+  agent_kill_session "$SESSION_NAME"
   # If we claimed the issue but never created a PR, unclaim it
   if [ "$CLAIMED" = true ] && [ -z "${PR_NUMBER:-}" ]; then
     log "cleanup: unclaiming issue (no PR created)"