Allow config.Save to clear explicitly reset configuration values

[Luqueee]

Problem

In internal/config/config.go:257-333, updateMapping only writes non-zero fields. Existing values therefore cannot be cleared or reset through a Save round-trip.

Examples include:

• ports and port lists remain after setting them to empty.
• admin_addr remains after disabling the admin API.
• insecure: true remains after setting it back to false.
• Empty string and zero-valued settings remain stale in the YAML.

This can make mole config edit or other config mutations disagree with the effective runtime configuration.

Acceptance criteria

• Define an explicit distinction between “field not requested” and “field requested with its zero value”.
• Make clearing ports, admin_addr, insecure, and other resettable fields deterministic.
• Preserve comments, key order, and user-only YAML keys.
• Add tests for clearing booleans, strings, slices, and numeric values.
• Keep the documented precedence between defaults, YAML, and CLI flags.