Replace the shell's crashed flag with an attempt status enum

[adamw]

What was wrong

• The shell passed a crashed: Boolean from the manifest reader to the picker, continue --list and the resume notice.
• If a crashed attempt's pid was reused by another process, the attempt no longer showed as crashed.

What changed

• New ObservedStatus enum: Running, Succeeded, Failed, Crashed. It is the manifest's status, except a Running attempt whose process is gone is Crashed.
• continue --list --json: the crashed boolean is replaced by attemptStatus (e.g. "Crashed").
• A live process that started more than a minute after the attempt's startedAt counts as a reused pid, so the attempt shows as crashed.