fix(vfs): provision /home/<username> for configured username

[chaliy]

What

When Bash::builder().username("eval") is configured, the default in-memory VFS now provisions /home/eval so scripts that write to $HOME / ~ succeed.

Why

Fixes #2128. The interpreter already sets HOME (and ~) to /home/<username> (Interpreter::with_config), but InMemoryFs only pre-creates /home/user. With a non-default username, the home directory did not exist, so:

let mut bash = Bash::builder().username("eval").build();
bash.exec("echo hi > /home/eval/x.sh").await   // io error: parent directory not found

This surfaced in an LLM eval run where file_path_organizer failed across all five models — each wrote its script under /home/eval/… and hit "parent directory not found".

How

• New sync helper InMemoryFs::add_dir(path, mode) (mirrors add_file's parent-creation; leaves existing entries untouched; respects VFS path limits).
• BashBuilder::build() now creates /home/<username> (mode 0o755, matching /home/user) on the default VFS when a username is configured. Custom filesystems remain responsible for their own layout.

Tests

• test_username_provisions_home_dir — echo hi > /home/eval/x.sh succeeds
• test_username_home_tilde_write — ~ / $HOME writes land in the provisioned home
• test_default_username_home_still_present — default /home/user unaffected
• add_dir doctest

cargo fmt --check and cargo clippy clean. Spec vfs.md updated.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	58c66ff	Commit Preview URL Branch Preview URL	Jun 28 2026, 01:41 AM