Add freshness scoring, auto-decay, and USAGE.md

Add lastAccessedAt timestamp to nodes with schema migration and
backfill. Touch timestamp on read, apply exponential freshness decay
(~69-day half-life) to search scoring alongside BM25 and vector
weights. Add auto-decay that marks untouched nodes as stale after a
configurable threshold, with CLI command and server-side daily interval.
Include comprehensive USAGE.md documenting all CLI commands and REST API.

portal/src/types.ts

@@ -14,6 +14,7 @@ export interface CortexNode {
   metadata: Record<string, any>;
   createdAt: number;
   updatedAt: number;
+  lastAccessedAt?: number;
   isStale?: boolean;
 }