feat(remember): full signal telemetry in Result.meta + per-node scores

[KailasMahavarkar]

Step 1 of the retrieval-observability effort. Makes `REMEMBER` self-describing so callers can see why a result looks the way it does without reading handler source. Foundation for Step 2 (`EXPLAIN REMEMBER`) and Step 3 (`ANSWER` verb).

Per-node scores (additive, no removals)

Every returned node carries:

Field	Meaning
`_remember_score`	fused final (or reranker score when rerank ran)
`_vector_sim`	max sentence cosine
`_bm25_score`	normalised FTS5 score
`_recency_score`	`exp(-age/half_life)`
`_graph_score`	new: normalised entity-degree contribution
`_co_bonus`	new: bonus when found by both vec and bm25
`_recall_boost`	new: `log1p(recall_count) * 0.05`
`_rank_stage`	new: `"fusion"` or `"rerank"`
`_fusion_score`	new: pre-rerank base, preserved when rerank ran
`_rerank_score`	new: raw reranker score, when rerank ran

Rich `Result.meta["signals"]` (always populated)

```python
{
"fusion": {"method": "weighted", "weights": [...], "graph_signal_enabled": True},
"recency": {"half_life_days": 7300.0},
"sentence_query_expansion": {"enabled": True, "num_sentences": 1},
"stages": {
"gathered_vec": int, "gathered_bm25": int,
"union": int, "cap_applied": bool,
"after_cap": int, "before_rerank": int,
"final": int,
},
"reranker": {"ran": bool, "model": str|None, "error": str|None},
"nucleus": {"enabled": bool},
}
```

Why this matters

We burned a session debugging "why does REMEMBER return 'Melanie' as content". Answer lived 200 lines deep in handler code. With this telemetry, the story is: look at `meta["signals"]["stages"]` + the `_*` fields. Done.

Tests

• `test_remember_includes_score_breakdown` updated to assert the full surface (prior comment claimed scores were removed in pipeline refactor - stale, they're back)
• `test_remember_meta_signals_telemetry` new - full meta shape + key presence
• `test_remember_graph_signal_reflected_in_meta` new - config drives meta

Full suite

```
1756 passed, 101 skipped
```

Zero regressions.

Next

Step 2: extend `SYS EXPLAIN REMEMBER "q"` to dry-run and emit the same telemetry without side effects. Step 3: add `ANSWER` verb that consumes the meta for attribution.

🤖 Generated with Claude Code