fix(cli): use workspace cron store in agent mode

[who96]

Summary

• Use the workspace-scoped cron store in nanobot agent
• Add a regression test for the CLI agent cron store path

Problem
Cron jobs created in CLI agent mode were stored in ~/.nanobot/cron/jobs.json while gateway mode used ~/.nanobot/workspace/cron/jobs.json, so jobs were not shared across modes.

Changes

• nanobot/cli/commands.py: align agent() cron storage with the workspace-scoped jobs.json path used by gateway()
• tests/test_commands.py: add a regression test that would fail under the old get_data_dir() path
  Fixes Cron jobs use two different storage locations (workspace vs config dir) #1649