fix(agent): run sandbox commands directly instead of via node's entrypoint

[radim10]

Summary

• A misspelled agent gave a misleading Node error. The default sandbox image inherited the node base image's entrypoint, which runs any command it can't find as a Node script, so agent run -- claude. failed with Error: Cannot find module '/workspace/claude.'.
• The image now clears the entrypoint (ENTRYPOINT []). The command is exec'd directly and a typo fails with exec claude. failed: No such file or directory (exit 127). Every command the image runs is a real executable: the agent, the network holder's sleep, and agent docker shell, which already passes its own --entrypoint bash.
• Existing installs keep the old image until it's rebuilt with stashbase agent docker build --force.

Test plan

• Unit test: the embedded Dockerfile clears the entrypoint
• cargo fmt --check and full cargo test pass
• Checked by hand on a Linux VM with the rebuilt image: the typo gives the exec error and exit 127; a normal run still starts the network holder and the agent