fix(interpreter): seed cd dash from OLDPWD exec option

[skovranek]

Summary

• seed the per-exec interpreter previousDir from OLDPWD when callers pass an env snapshot into exec()
• add regression coverage for cd - across isolated exec() calls using persisted cwd and env

Why

exec() supports per-call cwd and env, which lets callers preserve shell state between isolated executions. cd updates OLDPWD in the returned env, but cd - reads the interpreter's internal previousDir state instead.

That means a caller can pass the returned PWD and OLDPWD into the next exec() call and still get stale cd - behavior.

Verification

• pnpm --filter just-bash test:run src/Bash.exec-options.test.ts src/interpreter/builtins/cd.test.ts
• pnpm --filter just-bash test:run src/spec-tests/bash/spec.test.ts -t 'cd - without OLDPWD'
• pnpm lint
• pnpm knip
• pnpm --filter just-bash typecheck
• pnpm --filter just-bash build

[vercel]

@skovranek is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[cramforce]

Please sign your commit

[skovranek]

Please sign your commit

@cramforce Thank you, it's verified now.

I hadn't added the key to GH for signing so it couldn't verify the sig.

Ref for others:
https://docs.github.com/en/authentication/connecting-to-github-with-ssh/adding-a-new-ssh-key-to-your-github-account?tool=cli