spec: drop last_surfaced_at — not used in SBIA model

[toejough]

Problem

`last_surfaced_at` is referenced in the tracking step and recall operations but is not in the final schema. It was used by the frecency package for quality-weighted scoring, but the SBIA model dropped frecency — ranking is effectiveness-only.

Fix

Drop `last_surfaced_at` entirely. Remove references from:

• Surfacing tracking step (just write `surfaced_count` + `[[pending_evaluations]]`, not `last_surfaced_at`)
• Recall operations table (line 664)
• Any other references

Nothing in the SBIA design reads this field.

AI-Used: [claude]