Add `dew config` commands for viewing and managing global configuration

[vedanta]

Provide a first-class configuration experience.

Proposed:

dew config show
dew config path
dew config set <key> <value>
dew config unset <key>

Goals:

• Reduce manual editing.
• Improve discoverability.
• Provide a stable configuration surface.

Acceptance criteria:

• Show effective configuration.
• Display config file location.
• Support safe updates.
• Preserve backward compatibility.
• Tests included.