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

lib/args.js

@@ -12,9 +12,10 @@ Commands:
 
 Options:
   --source <path>      SOURCE directory (required)
-  --prev <path>        PREV directory (required)
-  --pend <path>        PEND directory (required)
-  --deltas <path>      DELTAS directory (required)
+  --base <path>        Sets --prev, --pend, --deltas as subdirs of base path
+  --prev <path>        PREV directory (default: <base>/previous)
+  --pend <path>        PEND directory (default: <base>/pending)
+  --deltas <path>      DELTAS directory (default: <base>/deltas)
   --backend <name>     Delta backend: zstd (default), xdelta3
   --config <file>      Load options from JSON config file (flags override)
   --dry-run            Print what would happen, execute nothing
@@ -26,6 +27,7 @@ export function parseArgs(argv) {
     args: argv,
     options: {
       source:    { type: 'string' },
+      base:      { type: 'string' },
       prev:      { type: 'string' },
       pend:      { type: 'string' },
       deltas:    { type: 'string' },

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]);