fix(sessions): compose verified Codex rollout continuations

[danshapiro]

Summary

• Treat verified Codex rollout files as parts of one session, using the session identity evidence in the files.
• Keep files separate when their identity is uncertain or conflicting, and preserve every path involved in a collision.
• Include continuation files when building the session directory and keep the History sidebar entry visible while its history loads.
• Add coverage for valid continuations, collisions and incomplete evidence, refresh behavior, and the browser session list.

Validation

• cargo fmt --all --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test --workspace --exclude freshell-tauri — passed
• pnpm run --silent typecheck — passed

All four checks ran from the rebased branch as part of the required pre-push hook.