fix: skip coldkey creation in wallet create when coldkey already exists

[shardi-b]

Description

Fixes #861 - btcli wallet create unconditionally attempts to create a new coldkey even when one already exists for the wallet, causing confusing behavior.

Changes Made

• Added a check in wallet_create() to verify if the coldkey file already exists before attempting creation
• When coldkey exists and --overwrite is not set, prints an informational message and skips to hotkey creation
• When --overwrite is explicitly passed, existing behavior is preserved (coldkey gets recreated)

Issue Link

• Fixes: wallet create: unconditionally creates coldkey even when one already exists #861

Testing

Manual Testing

1. Created a wallet: btcli wallet create --wallet-name test-wallet
2. Ran same command to add new hotkey: btcli wallet create --wallet-name test-wallet --wallet-hotkey new-hotkey
3. Before fix: Shows coldkey mnemonic, prompts to overwrite coldkey file, errors if declined
4. After fix: Prints "Coldkey already exists, skipping coldkey creation", then only creates the hotkey

Test Results: Fix verified manually - coldkey creation is skipped, only hotkey mnemonic shown, no overwrite prompt.

Automated Testing

Test Command(s):

# Existing e2e tests in tests/e2e_tests/test_wallet_creations.py cover wallet creation flows

Root Cause

In bittensor_cli/src/commands/wallets.py, the wallet_create function called wallet.create_new_coldkey() unconditionally without checking if a coldkey already existed:

# Before (buggy)
wallet.create_new_coldkey(...)  # Always runs, even if coldkey exists
wallet.create_new_hotkey(...)

# After (fixed)  
if wallet.coldkey_file.exists_on_device() and not overwrite:
    console.print("Coldkey already exists, skipping...")
else:
    wallet.create_new_coldkey(...)
wallet.create_new_hotkey(...)

This pattern (coldkey_file.exists_on_device()) is already used elsewhere in the codebase (e.g., line 604, 873) for the same purpose.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• New and existing unit tests pass locally with my changes