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.

src/core/store.ts

@@ -16,6 +16,7 @@ function rowToNode(row: any): Node {
     embedding: row.embedding ? deserializeEmbedding(row.embedding) : null,
     createdAt: row.created_at,
     updatedAt: row.updated_at,
+    lastAccessedAt: row.last_accessed_at ?? row.updated_at,
     isStale: !!row.is_stale,
   };
 }
@@ -36,13 +37,13 @@ export async function addNode(input: AddNodeInput): Promise<Node> {
   const embedding = await getEmbedding(`${input.title} ${content}`);
 
   db.prepare(`
-    INSERT INTO nodes (id, kind, title, content, status, tags, metadata, embedding, created_at, updated_at)
-    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+    INSERT INTO nodes (id, kind, title, content, status, tags, metadata, embedding, created_at, updated_at, last_accessed_at)
+    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
   `).run(
     id, input.kind, input.title, content, input.status ?? null,
     JSON.stringify(tags), JSON.stringify(metadata),
     embedding ? serializeEmbedding(embedding) : null,
-    now, now
+    now, now, now
   );
 
   // Insert tags
@@ -53,14 +54,16 @@ export async function addNode(input: AddNodeInput): Promise<Node> {
 
   return {
     id, kind: input.kind, title: input.title, content, status: input.status,
-    tags, metadata, embedding, createdAt: now, updatedAt: now, isStale: false,
+    tags, metadata, embedding, createdAt: now, updatedAt: now, lastAccessedAt: now, isStale: false,
   };
 }
 
 export function getNode(id: string): Node | null {
   const db = getDb();
   const row = db.prepare('SELECT * FROM nodes WHERE id = ?').get(id) as any;
-  return row ? rowToNode(row) : null;
+  if (!row) return null;
+  db.prepare('UPDATE nodes SET last_accessed_at = ? WHERE id = ?').run(Date.now(), id);
+  return rowToNode(row);
 }
 
 export function findNodeByPrefix(prefix: string): Node | null {