Add --base meta-argument to set prev/pend/deltas as subdirs

lib/config.js

@@ -3,6 +3,7 @@
  * CLI args always win. Required paths are validated here.
  */
 import { readFile } from 'fs/promises';
+import { join } from 'path';
 
 const REQUIRED_PATHS = ['source', 'prev', 'pend', 'deltas'];
 const DEFAULTS = {
@@ -25,6 +26,13 @@ export async function loadConfig(args) {
   // CLI args override file config, file config overrides defaults
   const config = { ...DEFAULTS, ...fileConfig, ...filterDefined(args) };
 
+  // Expand --base into --prev/--pend/--deltas, explicit flags take priority
+  if (config.base) {
+    config.prev   ??= join(config.base, 'previous');
+    config.pend   ??= join(config.base, 'pending');
+    config.deltas ??= join(config.base, 'deltas');
+  }
+
   // Guard: refuse to run if any required path is missing
   if (config.command === 'run') {
     const missing = REQUIRED_PATHS.filter(k => !config[k]);