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/helpers.ts

@@ -28,6 +28,8 @@ export function testHooks(over: Partial<AppHooks> = {}): AppHooks {
     applyProvider: async () => 'configured',
     listModels: async () => ({ models: [] }),
     listSessions: async () => 'no saved sessions',
+    searchSessions: async () => '',
+    forkSession: async () => 'forked',
     listSkills: () => 'no skills loaded',
     listPlugins: () => 'no plugins active',
     listMemory: async () => 'nothing remembered yet',