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

docs/architecture.md

@@ -247,7 +247,7 @@ the reasoning.
 | `tools.ts` | file and shell tools, tool sets, ripgrep bridge, bash streaming and interrupt |
 | `tools-git.ts` | read-only git tools, spawned with a fixed argv |
 | `commit.ts` | `git_commit_message`, a nested model call over the staged diff |
-| `tools-net.ts` | `web_fetch`, private-address and redirect checks |
+| `tools-net.ts` | `web_fetch`, `web_search`, private-address and redirect checks |
 | `ignore.ts` | gitignore-aware walker, path jail |
 | `complete.ts` | `@path` token extraction, ranking, insertion |
 | `registry.ts` | external index, validation, install and removal |