feat(snapshot): accept instance name and auto-detect volumes

[ivyleavedtoadflax]

Summary

Closes #87. snapshot create now accepts an instance name to auto-detect attached volumes instead of requiring --volume-id.

• Add positional instance_name argument to snapshot create (follows codebase convention for instance params)
• Add --device / -D option to filter to a specific device (e.g., /dev/sdf)
• --volume-id remains supported as the alternative single-volume path
• When multiple volumes are found, snapshot names are auto-suffixed with the device name (e.g., backup-sda1, backup-sdf)
• Partial failure handling: if one volume fails, remaining volumes are still snapshotted, with a summary at the end
• Add get_volumes_for_instance utility; refactor get_volume_ids to delegate to it (DRY)

Usage

# Snapshot all volumes for an instance
remote snapshot create my-instance -n my-snapshot

# Snapshot only the data volume
remote snapshot create my-instance --device /dev/sdf -n my-snapshot

# Existing single-volume usage still works
remote snapshot create -v vol-123456 -n my-snapshot

Test plan

• 27 snapshot tests pass (13 new, 14 existing)
• Full suite: 1135 passed, 1 skipped
• Ruff lint and format clean
• Verified: single-volume path (--volume-id) unchanged
• Verified: instance with single volume uses name as-is
• Verified: instance with multiple volumes suffixes names with device
• Verified: --device filters correctly
• Verified: partial failure reports successes and failures
• Verified: mutually exclusive options error correctly
• Verified: instance resolution errors handled

https://claude.ai/code/session_018cz4V49XUdte4NBJfGR21S