Cleanup: retire /proc actual_backend; deterministic image-tag mismatch

[thinmintdev]

Parent

#652

What to build

For container slots, drop the fragile /proc//exe actual_backend introspection. The running backend IS the image tag (podman inspect); 'mismatch' = running image tag ≠ slot's profile image. Update the dashboard mismatch chip accordingly.

Acceptance criteria

• No /proc introspection for container slots
• actual_backend = image tag via podman inspect; mismatch computed from it
• lemond slots keep existing behavior until they migrate

Blocked by

• Dashboard: slot-card container variant (read-only) #657
• P2 live cutover: agent + chat to containers on CT105 #662

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