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

test/commands.test.ts

@@ -82,7 +82,7 @@ test('a lone slash lists the whole menu', () => {
 
 test('a prefix narrows the menu', () => {
   expect(matchCommands('/co').map((c) => c.name)).toEqual(['context', 'compact', 'cost']);
-  expect(matchCommands('/se').map((c) => c.name)).toEqual(['sessions']);
+  expect(matchCommands('/se').map((c) => c.name)).toEqual(['sessions', 'search']);
   expect(matchCommands('/ag').map((c) => c.name)).toEqual(['agent']);
   expect(matchCommands('/th').map((c) => c.name)).toEqual(['think']);
 });