fix(container-runner): raise install_packages build timeout to 15min

[Shlomog]

Summary

• Raises the execSync timeout in buildAgentGroupImage() from 5 minutes (300_000 ms) to 15 minutes (900_000 ms).
• Surfaced via the install_packages self-mod flow: a real install with apt + npm packages on a slow network hit spawnSync /bin/sh ETIMEDOUT mid-build, leaving the agent group's container image not rebuilt and the pending install silently abandoned.
• The previous timeout was tight specifically for first-time package installs — that's the only path that exercises this code, so the worst-case is also the common case here.

Test plan

• Trigger an install_packages approval that adds at least one apt package (e.g. ffmpeg) on a fresh image, verify build completes without ETIMEDOUT.
• Verify the original guard still fires for a genuinely hung build (kill dockerd mid-build → expect timeout after 15 min, not silent hang).

🤖 Generated with Claude Code

[gavrielc]

Thanks for the contribution! Clean fix — appreciated.