feat: implement provider-side prompt caching and workflow scaffolding

- Added support for caching the stable prefix of the system prompt for Anthropic models.
- Introduced a new option `cacheSystemPrefix` in SessionOptions to enable caching.
- Implemented a nudge escalation system that reminds users to update the TODO.md file, capped at three nudges.
- Created a new `scaffoldWorkflowFiles` function to generate TODO.md, ROADMAP.md, and docs/ directory when they are missing.
- Updated the App component to scaffold workflow files during initialization.
- Added hooks functionality to allow external scripts to modify tool input and manage approvals.
- Implemented tests for the new hooks functionality, ensuring proper approval and execution flow.

test/helpers.ts

@@ -55,6 +55,7 @@ export function testHooks(over: Partial<AppHooks> = {}): AppHooks {
       remove: async (name) => `removed ${name}`,
     },
     initPrompt: 'write AGENTS.md',
+  scaffoldWorkflow: () => [],
     history: [],
     recordPrompt: () => {},
     ...over,