Add js_repl cwd and homeDir helpers

[fjord-oai]

Summary

This PR adds two read-only path helpers to js_repl:

• codex.cwd
• codex.homeDir

They are exposed alongside the existing codex.tmpDir helper so the REPL can reference basic host path context without reopening direct process access.

Implementation

• expose codex.cwd and codex.homeDir from the js_repl kernel
• make codex.homeDir come from the kernel process environment
• pass session dependency env through js_repl kernel startup so codex.homeDir matches the env a shell-launched process would see
• keep existing shell HOME population behavior unchanged
• update js_repl prompt/docs and add runtime/integration coverage for the new helpers

[fjord-oai]

@codex review

[chatgpt-codex-connector]

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