allow $schema key in config for IDE integration

Users can now add "$schema": "./schemas/clawdbot.schema.json" to their
clawdbot.json for autocomplete without validation errors.

schemas/clawdbot.d.ts

@@ -6,6 +6,7 @@
  */
 
 export interface ClawdbotConfig {
+  $schema?: string;
   meta?: {
     lastTouchedVersion?: string;
     lastTouchedAt?: string;

schemas/clawdbot.schema.json

@@ -2,6 +2,9 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "type": "object",
   "properties": {
+    "$schema": {
+      "type": "string"
+    },
     "meta": {
       "type": "object",
       "properties": {