fix(handler): attribute agent CLI actions to agent identity

When agents use the multica CLI during task execution, their comments,
issue updates, and issue creations were attributed to the daemon's user
(via JWT) instead of the agent. Pass MULTICA_AGENT_ID env var from the
daemon, send X-Agent-ID header from the CLI client, and use it in
handlers to set the correct author/actor identity.

server/internal/daemon/daemon.go

@@ -662,6 +662,7 @@ func (d *Daemon) runTask(ctx context.Context, task Task, provider string) (TaskR
 		"MULTICA_DAEMON_PORT":  fmt.Sprintf("%d", d.cfg.HealthPort),
 		"MULTICA_WORKSPACE_ID": d.workspaceIDForRuntime(task.RuntimeID),
 		"MULTICA_AGENT_NAME":   agentName,
+		"MULTICA_AGENT_ID":     task.AgentID,
 		"MULTICA_TASK_ID":      task.ID,
 	}
 	// Point Codex to the per-task CODEX_HOME so it discovers skills natively