Fix -reset command to properly wipe all node data

[AlanRuno]

Summary

The -reset command was not properly wiping blockchain data, DHT peer table, and rapid stake table data because it was looking in the wrong directory (build/lib/data) instead of the actual data directory (./data) where the node stores all persistent data.

This PR fixes the issue by:

1. Correcting the default data directory path from build/lib/data to ./data (matching the config's data_dir setting)
2. Simplifying the reset function to remove the entire data directory instead of individual files, ensuring ALL data is wiped including node identity keys

Review & Testing Checklist for Human

• Verify the data directory path is correct: Confirm that ./data is indeed where the node stores its data when running from the mxdlib root directory (check default_config.json has data_dir: "./data")
• Test standalone reset: Run ./letsgo -reset and verify it removes the ./data directory contents
• Test reset with network type: Run ./letsgo testnet -reset and verify the node starts fresh with a new identity after the reset
• Verify node regenerates keys: After reset, confirm the node creates new identity keys in data/node_keys.v2 on startup

Recommended Test Plan

1. On a test node with existing data, run ls -la data/ to see current contents
2. Run ./letsgo -reset (answer 'y' to confirm)
3. Verify the data directory is now empty (except for the directory itself)
4. Run ./letsgo testnet and verify the node starts fresh and regenerates its identity

Notes

• The reset now also deletes node identity keys (data/node_keys.v2), which will be regenerated on next startup. This is intentional per the "wipe everything" requirement.
• The old approach tried to delete specific files like blockchain.db, blocks.db, peers.db which don't actually exist - all data is stored in utxo.db within the data directory.

Link to Devin run: https://app.devin.ai/sessions/02b7aca7d4b047d2911c624c278ccaaa
Requested by: Runo (@AlanRuno)

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