docs(workers): document per-operation slot reservations (#1199)

[r266-tech]

Summary

#1199 (merged 2026-04-22 by @nicoloboschi) added three new worker-slot reservation env vars and introduced a shared-pool overflow model, but hindsight-docs/docs/developer/configuration.md still only documents the pre-#1199 two-slot-pool model. Users following the docs won't know they can reserve slots for retain / file_convert_retain / refresh_mental_model, and the admonition's claim that "retain and other non-consolidation tasks may use at most 10 − 2 = 8 concurrent slots" no longer reflects the new semantics.

This PR:

• Extends the Distributed Workers table with the three new env vars and their defaults.
• Rewrites the slot-reservation admonition to describe the per-operation reservations + shared pool + overflow behavior + the validation rule (sum of reservations > WORKER_MAX_SLOTS raises ValueError at startup).
• Mirrors the change into skills/hindsight-docs/references/developer/configuration.md.

Values verified against hindsight-api-slim/hindsight_api/config.py:

• WORKER_SLOT_RESERVATION_TYPES dict (defaults: consolidation=2, retain=0, file_convert_retain=0, refresh_mental_model=0).
• Validation total_reserved > self.worker_max_slots raising ValueError with message "Reduce reservations or increase HINDSIGHT_API_WORKER_MAX_SLOTS".

Pure docs, no behavior change.