Add storage diagnostics to `codex doctor` (#38795)

## What changed

- Report available space for `CODEX_HOME` and the active worktree, warning below 5 GiB and failing below 1 GiB.
- On Windows, report whether the active Git worktree is on a trusted Dev Drive and provide remediation when it is not.
- Run attached doctor reports against the conversation workspace, including workspaces recovered from persisted thread metadata, while keeping the subprocess anchored in `CODEX_HOME` so deleted workspaces do not prevent it from starting.

## Testing

- Cover disk thresholds, measurement failures, missing paths, conversation workspace selection, command construction, Windows volume detection, and human-readable output.

GitOrigin-RevId: 0a99f9d278717e7d19936ffc504984b3a0958c25