Add `dew doctor --all` to validate every image managed under ~/.dew

[vedanta]

Extend doctor to operate globally.

Proposed:

dew doctor --all

Capabilities:

• Enumerate all images.
• Verify decryptability.
• Detect missing ownership markers.
• Detect stale or corrupt images.
• Produce a summary report.

Acceptance criteria:

• Current behavior remains unchanged without --all.
• Global summary includes healthy and unhealthy counts.
• Corrupt images are clearly identified.
• Tests included.