Clarify resume guidance in exit summaries

[copyberry]

Clarify resume guidance in exit summaries

What changed

• Show the exact codex resume <thread-id> command on its own indented line.
• When a thread has a name, also explain that users can run codex resume and select the named thread from the picker.
• Apply command highlighting to both resume options when color is enabled.

Testing

• Cover unnamed, empty-name, named, colored, and embedded-session exit summaries.

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