Add `dew remote doctor` for remote connectivity and permission diagnostics

[vedanta]

Add a dedicated remote diagnostics command.

Proposed:

dew remote doctor

Checks:

• destination configured
• destination syntax valid
• ssh/scp availability
• remote path reachable
• remote path writable
• image visibility
• pull/push preflight validation

Acceptance criteria:

• Produces actionable remediation guidance.
• Read-only by default.
• Works with local and remote destinations.
• Tests included.