config: update AccountConfig `Unlock` to be of type `[]int`

## Task summary

- `AccountConfig.Unlock` should be turned into type `[]int`, representing indices of accounts in the keystore directory to be unlocked
- `keystore.UnlockKeys()` and `cmd/gossamer/unlockKeystore()` will need to be updated respectively

The task names AccountConfig.Unlock, keystore.UnlockKeys(), and cmd/gossamer/unlockKeystore(); start by tracing how the config value reaches the keystore and command entry point. Update the affected paths so account indices are handled consistently, then verify that the affected Go code builds and the unlock behavior matches the requested []int representation.

Written by the indexing model from the issue text.