Add `dew key backup` and `dew key import` for identity disaster recovery

[vedanta]

Add first-class identity backup and recovery workflows.

Proposed:

dew key backup <path>
dew key import <path>
dew key export --public

Goals:

• Prevent accidental identity loss.
• Provide an officially supported backup path.
• Reduce manual file copying.

Acceptance criteria:

• Explicit warnings before writing private key material.
• Backup file permissions are restricted.
• Import validates the key before installation.
• Existing identity replacement requires confirmation or force flag.
• Tests cover backup, import, invalid key, and overwrite scenarios.