v0.2.4 — Preserve forked sessions

Fixes

• Fix forked sessions disappearing from the session list when inherited session_meta records overwrite the fork's identity (#3).
• Preserve the ID, working directory, and creation time from the first session_meta record while continuing to read later activity. Nested forks remain separate sessions.
• Prevent parent and fork rollout files from being incorrectly grouped together for deletion. Previously, deleting the merged entry could also remove related fork files.
• Add regression tests for fork metadata, nested forks, missing IDs, and independent deletion of parents and forks.

Users who use session forks should update before deleting sessions through the TUI. This fix does not restore files that were already deleted.

Thanks to @mhjacobson for reporting the problem and suggesting the initial fix.

Validation: go test ./..., go test -race ./..., go vet ./..., and go build -buildvcs=false ./....

Full changelog: v0.2.3...v0.2.4