feat: add transcript/history CLI commands

[justinmerrell]

Summary

Add CLI commands for managing session transcripts/history.

Commands to implement

• musher history list — list stored transcript sessions with timestamps and metadata
• musher history view <id> — view transcript events for a specific session
• musher history prune — delete sessions older than a specified duration (e.g. --older-than 30d)

Current state

musher-cli has internal/transcript/ with store and transcript types, but no CLI commands to access them.

Reference source (mush)

• cmd/mush/history.go — CLI commands for list/view/prune
• internal/transcript/ — store and transcript data types