Allow resizing hibernated droplets by modifying the size tag

[Ninja3047]

Summary

• Resize hibernated snapshots — dropkit resize <name> now works on hibernated droplets by swapping the size: tag on the snapshot, so the next wake creates the droplet with the new size. No need to wake, resize, and re-hibernate.
• New untag_resource API method — adds DELETE /tags/{tag}/resources support to complement the existing tag_resource.
• Improved find_user_droplet return type — always returns the username (never None), simplifying error handling across 8+ call sites that no longer need username null checks.
• Better hibernated listing — dropkit list now shows both the droplet size slug and image size for hibernated snapshots.
• Housekeeping — removes accidentally committed .coverage file, adds it to .gitignore.

How it works

When resizing a hibernated snapshot:

1. Reads the current size from the size:<slug> tag on the snapshot
2. Validates the new size against available DO sizes
3. Atomically swaps the tag: creates and applies the new size:<new> tag first, then removes the old size:<old> tag (crash-safe ordering)

Test plan

• Unit tests for _resize_hibernated_snapshot (no ID, no size tag, same size, successful swap, cancelled, invalid size)
• Unit tests for untag_resource API method
• Unit tests for complete_droplet_or_snapshot_name (combines sources, deduplicates, ordering)
• Manual: dropkit resize <hibernated-name> prompts interactively and updates tag
• Manual: dropkit resize <hibernated-name> -s s-2vcpu-4gb updates tag non-interactively
• Manual: dropkit list shows droplet size column for hibernated snapshots
• Manual: dropkit wake <name> uses updated size after resize

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