fix: expose INIT_CWD to tasks

[wyf027]

Summary

• Set INIT_CWD from the directory where vt was invoked before task planning/execution.
• Include INIT_CWD in the default untracked env passthrough set so cached tasks can read it.
• Add an e2e fixture that runs a cached task from packages/app/src and verifies the task process receives that invocation directory even though the task itself runs from packages/app.

Closes #243.

Tests

• cargo fmt --all --check
• cargo test -p vite_task_bin --test e2e_snapshots init_cwd
• cargo test -p vite_task_graph