fix(tui): show session timestamps in listings

[cyq1017]

Problem

• Session listings only showed relative ages, which makes it hard to find a specific older conversation by date.
• The workspace-scoped resume paths also needed a clearer pointer that --last/--continue choose the current workspace, while explicit ids can resume other directories.

Change

• Show an absolute UTC last-updated timestamp alongside the relative age in codewhale sessions and the TUI session picker.
• Document the difference between all-session listing and current-workspace resume paths.

Verification

• cargo test -p codewhale-tui absolute_updated_timestamp --all-features --locked -- --nocapture
• cargo test -p codewhale-tui session_picker --all-features --locked -- --nocapture
• cargo fmt --all -- --check
• cargo check -p codewhale-tui --all-features --locked
• git diff --check

Refs #2494

[gemini-code-assist]

Warning

You have reached your daily quota limit. Please wait up to 24 hours and I will start processing your requests again!

[greptile-apps]

cyq1017 has reached the 50-review limit for trial accounts. To continue receiving code reviews, upgrade your plan.