MUL-6640: fix stale Autopilot subscribers after member removal (#7501)

* fix(autopilot): prune departed subscribers

Co-authored-by: multica-agent <github@multica.ai>

* fix(autopilot): serialize subscriber saves with revocation

Co-authored-by: multica-agent <github@multica.ai>

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Co-authored-by: Sol-Boy <sol-boy@multica-ai.local>
Co-authored-by: multica-agent <github@multica.ai>