fs/session: fix deadlock in agent/new via setAgentLogLocked

SetAgentLog acquires sess.mu, but CreateAgent in create.go
already holds sess.mu when calling it — causing a deadlock on
agent/new writes. Split into SetAgentLog (locks) and
setAgentLogLocked (no lock) so callers that already hold the
lock can use the locked variant directly.

fs/session/create.go

@@ -264,12 +264,12 @@ func CreateAgent(rs *rootState, sessName string, args []string) error {
 		ReadPlanStep: func() string {
 			return ""
 		},
-	})
+})
 
 	sess.mu.Lock()
 	sess.Core = core
 	sess.uname = uname
-	sess.SetAgentLog(uname, al)
+	sess.setAgentLogLocked(uname, al)
 	sess.proc = proc
 	sess.mu.Unlock()
 

fs/session/types.go

@@ -79,6 +79,11 @@ func (sess *Session) AgentLog() *AgentLog {
 func (sess *Session) SetAgentLog(agentID string, al *AgentLog) {
 	sess.mu.Lock()
 	defer sess.mu.Unlock()
+	sess.setAgentLogLocked(agentID, al)
+}
+
+// setAgentLogLocked is like SetAgentLog but assumes sess.mu is already held.
+func (sess *Session) setAgentLogLocked(agentID string, al *AgentLog) {
 	if sess.AgentLogs == nil {
 		sess.AgentLogs = make(map[string]*AgentLog)
 	}