[codex] fix root workspace guard

[flyzstu]

Summary

• allow projects under the user's home directory instead of rejecting the entire home subtree
• keep blocking direct access to the home directory itself, filesystem root, and high-risk system directory trees
• add regression coverage for the home-directory case

Root cause

The 0.1.2 security hardening treated /root and other home directories as forbidden trees, so running mimo from /root/ failed during Instance.provide initialization.

Validation

• static review of startup directory guards and related trust checks
• added regression test coverage for home vs home subdirectory behavior
• attempted local bun test and bun typecheck, but the environment is missing workspace dependencies (@opentui/solid/preload, tsgo)