feat(observability): measure private-memory retrieval quality without memory content

[robinbraemer]

Outcome

Measure private-memory extraction, consolidation, retrieval quality, index health, and attachment amplification without exporting memory content.

Scope

• Add bounded signals for extraction, Dream, retrieval method, candidates, selected topics/chunks, attachment bytes/tokens, latency, fallback, and degradation reason.
• Record index schema/model freshness, rebuild outcomes, corruption recovery, and forget invalidation.
• Link auxiliary provider calls to the owning Agent operation where possible.
• Define offline evaluation metrics separately from production operational metrics.

Acceptance criteria

• Operators can distinguish selector, lexical, vector, hybrid, and fallback paths.
• Retrieval amplification and auxiliary model cost can be correlated to the main operation.
• Topic paths, scopes, snippets, bodies, queries, embeddings, and model inputs are absent from exported telemetry.
• Pause/forget telemetry proves invalidation occurred without revealing what was forgotten.
• Memory instrumentation remains fail-open for the main turn.