feat(store): add recall tracking and promoted observations endpoint

[sergiomarquezdev]

Feature Request

Problem

Engram tracks write-side metrics (revision_count, duplicate_count, last_seen_at) but has zero read-side instrumentation. There is no way to know which observations are frequently recalled by agents across sessions.

This means all observations are treated equally — a fact recalled 50 times has no more visibility than one never accessed.

Proposed Solution

Add recall tracking to observations and expose a /promoted endpoint (+ mem_promoted MCP tool) that surfaces frequently-recalled observations.

Store changes:

• Add recall_count INTEGER NOT NULL DEFAULT 0 and last_recalled_at TEXT columns to observations
• Increment recall_count on Search(), GetObservation() (fire-and-forget, never blocks reads)
• New PromotedObservations() method: returns observations ordered by recall_count DESC with configurable threshold

HTTP API:

• GET /promoted?project=X&scope=Y&min_recalls=5&limit=7 — returns observations that have proven their value through repeated access

MCP tool:

• mem_promoted — agent-profile tool for surfacing high-value context at session start

Motivation

Inspired by NeoVertex1/nuggets which uses a "memory promotion" concept where frequently-accessed facts graduate to permanent system prompt context. This feature provides the data layer that clients can use to implement similar promotion strategies.

Implementation Notes

• incrementRecall() is fire-and-forget (db.Exec without error propagation) — recall tracking must never break reads
• Recall data is analytics metadata, not content — no need for enqueueSyncMutationTx
• All existing scan sites (8 total) need the new columns added to maintain consistency