[codex] scan session files until project cwd is found

[chaochaoweb3]

Summary

• scan each Claude session JSONL file until a non-empty cwd entry is found instead of stopping after the first 10 lines
• add a regression test where metadata entries push cwd past that previous 10-line limit

Why

The existing fallback still decodes ~/.claude/projects/<encoded> directory names when cwd is not found quickly. That decode is ambiguous for paths with dashes or underscores, which is the same failure mode discussed in #465 and later reports on #444.

Validation

• git diff --check
• equivalent scan sanity check with Python

I could not run the Rust test locally because this machine does not currently have cargo or rustfmt installed in PATH.