feat(search): optional time-decay re-ranking with durable exemption (#337)

[tuyenlethrive]

What

Implements optional time-decay re-ranking for search — roadmap item #337 ("recent memories surface before older ones").

When enabled, search_memories multiplies each hit's similarity by an Ebbinghaus half-life factor 0.5 ^ (age_days / half_life) (age from the drawer's filed_at), re-ranks, and truncates. A slightly weaker-matching recent memory can now outrank a stale stronger match.

Off by default

decay.enabled defaults to false — behavior is byte-identical unless turned on. All logic lives at the single retrieval choke point (search_memories), so the MCP search tool inherits it.

Config (~/.mempalace/config.json)

"decay": {
  "enabled": true,
  "half_life_days": 60,
  "durable_wings": ["memory", "identity"],
  "durable_rooms": []
}

Durable exemption

Drawers in durable_wings / durable_rooms, or tagged type: "durable" / "wisdom", skip decay (freshness forced to 1.0) so canonical notes don't sink. Exempting by wing is recommended over room, since room names are auto-assigned by the miner (a bulk import auto-labeled architecture should not be treated as canonical).

Caveats

• Decay re-ranks within an over-fetched candidate window (n_results × 6, capped) — it reorders near-matches, it does not rescue an item far outside the similarity top-N.
• ChromaDB can't decay server-side, so this is a client-side over-fetch + re-rank. No reindex; no schema change; missing filed_at → no decay (back-compat).

Testing

Validated before/after on a ~8k-drawer palace: recent tickets lift, stale bulk content sinks, curated memory/* notes stay put. Decay math + exemption covered by a self-check.

Files: mempalace/searcher.py, mempalace/config.py, mempalace/mcp_server.py (~90 lines).