fix: use virtual cwd with RealFSProvider to prevent path doubling

[ivan-tymoshenko]

When using RealFSProvider (enabled via fsRootPath), relative paths passed to agent tools resolve against process.cwd() before reaching the provider. If process.cwd() overlaps with the provider root (e.g. both are
/workspace inside a sandbox), the provider prepends the root again, creating doubled paths like /workspace/workspace/foo.txt.

Additionally, the just-bash tool defaults its cwd to /home/user, causing the model to construct absolute paths like /home/user/foo.txt that map to /workspace/home/user/foo.txt — wrong and inconsistent with the VFS root.

Fixes:

• Enable virtualCwd on the VFS and set it to / when fsRootPath is configured. This makes all relative paths resolve against / instead of process.cwd(), so foo.txt becomes /foo.txt which the provider correctly maps to
  {root}/foo.txt.
• Pass cwd: '/' to the just-bash constructor so pwd returns / and the model uses consistent paths.