feat(cli): add session prune command for cleanup

[dzianisv]

Summary

Adds opencode session prune <duration> command to delete old sessions and free disk space.

After heavy usage, the ~/.local/share/opencode/storage/ directory can grow to 10+ GB with thousands of old sessions. This command provides a safe way to clean up old data.

Features

• Duration format: 1d, 7d, 2w, 1m (days/hours/weeks/months)
• --dry-run: Preview what would be deleted without making changes
• --force/-f: Skip confirmation prompt
• --all/-a: Prune sessions from all projects (not just current)
• --logs: Also prune old log files
• --snapshots: Also prune orphaned snapshot directories

Usage Examples

# Preview cleanup of sessions older than 7 days
opencode session prune 7d --dry-run

# Clean up sessions older than 1 month with logs
opencode session prune 1m --logs

# Clean up everything older than 2 weeks, including orphaned snapshots
opencode session prune 2w --logs --snapshots --force

Output

Shows a summary of sessions to be deleted with age, estimated space savings, and asks for confirmation before proceeding.

Testing

Tested manually with --dry-run on a storage directory with 400+ sessions.

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