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/cli/index.ts

@@ -9,6 +9,7 @@ import { updateCommand } from './commands/update';
 import { removeCommand } from './commands/remove';
 import { graphCommand } from './commands/graph';
 import { serveCommand } from './commands/serve';
+import { decayCommand } from './commands/decay';
 import { closeDb } from '../core/db';
 
 const program = new Command();
@@ -27,6 +28,7 @@ program.addCommand(updateCommand);
 program.addCommand(removeCommand);
 program.addCommand(graphCommand);
 program.addCommand(serveCommand);
+program.addCommand(decayCommand);
 
 program.hook('postAction', () => {
   closeDb();