features: /changes, /search, /fork, web_search, prompt memoization, per-turn spend cap, workspace refresh

- /changes diffs the last turn's file snapshot (added/modified/deleted), reusing undo infra via SnapshotStack.peek()
- system prompt memoized behind version counters (notebook/memory/skills/plugins/tools/workspace); hit-rate in /cost, foundation for provider caching
- web_search: DuckDuckGo Lite, keyless, 5 results, SSRF-filtered, in the net set with ask permission
- /search <query>: full-text grep over saved sessions incl. tool-input JSON
- workspace file list re-walks at a turn boundary after writes
- maxSpendPerTurn: per-turn cap stops a runaway step with a notice
- /fork: branch the session at the last turn boundary, original untouched

821 tests pass, typecheck clean, build green

src/commands.ts

@@ -28,6 +28,9 @@ export type CommandAction =
   | { type: 'resume'; id: string }
   | { type: 'undo' }
   | { type: 'redo' }
+  | { type: 'changes' }
+  | { type: 'search'; query: string }
+  | { type: 'fork' }
   /** A custom command from a markdown file, expanded against its arguments. */
   | { type: 'custom'; command: CustomCommand; args: string[] }
   | { type: 'unknown'; name: string };
@@ -65,6 +68,9 @@ export const COMMANDS: CommandSpec[] = [
   { name: 'save', summary: 'write the session to disk now' },
   { name: 'undo', summary: 'undo the last turn — restores files and conversation (bash effects are not snapshotted)' },
   { name: 'redo', summary: 'redo the last undone turn' },
+  { name: 'changes', summary: 'show what the last turn changed on disk' },
+  { name: 'search', arg: '<query>', summary: 'search saved sessions for a phrase' },
+  { name: 'fork', summary: 'fork the session at the last turn boundary (keeps the original)' },
   { name: 'clear', summary: 'clear the transcript and history' },
   { name: 'exit', aliases: ['quit'], summary: 'quit' },
 ];
@@ -233,6 +239,12 @@ export function parseCommand(raw: string, custom: readonly CustomCommand[] = [])
       return { type: 'undo' };
     case 'redo':
       return { type: 'redo' };
+    case 'changes':
+      return { type: 'changes' };
+    case 'search':
+      return arg ? { type: 'search', query: arg } : { type: 'info', text: 'usage: /search <query>' };
+    case 'fork':
+      return { type: 'fork' };
     default: {
       const cmd = custom.find((c) => c.name === name);
       return cmd ? { type: 'custom', command: cmd, args: arg ? arg.split(/\s+/) : [] } : { type: 'unknown', name };