[codex] Fix fork --last cwd filtering

[canvrno-oai]

Fixes #20945.

This keeps codex fork --last aligned with the neighboring latest-session lookup flows. The local fork path now uses the same cwd-scope helper as resume --last, which is also a small code cleanup around how this selection logic is shared.

Credit to @chanwooyang1 for the report and for pointing out the narrow fix direction.

What changed:

• Route fork --last through the shared latest-session cwd filter.
• Preserve --all as the explicit opt-in for global latest-session selection.
• Keep remote cwd override behavior unchanged.
• Add focused coverage for local default, --all, and remote override filter semantics.

Validation:

• Ran just fmt.
• Ran git diff --check.
• Reviewed the fork --last, resume --last, and fork picker selection paths against the issue report.

[fcoury-oai]

Approved