refactor: add type-safe command interface to appInvoke

[whitelonng]

Summary

Fixes type safety issue where appInvoke used unsafe string cmd and any args, allowing typos, wrong argument types, and incorrect return types to pass type checking.

Changes

Type System

• Add AppInvokeCommands interface defining all commands with typed args and results
• Update appInvoke signature to use conditional types for type-safe invocation
• Command names are now autocompleted and typo-checked at compile time
• Argument structure is validated for each command
• Return types are inferred automatically, no manual type annotations needed

Code Updates

• Remove all generic type parameters from appInvoke call sites (33 locations across 7 files)
• Add savedAt field to list_agent_sessions result type to match actual schema
• Update internal implementation to use typed cmdArgs instead of loose args

Example

Before (unsafe):

const result = await appInvoke<any>('list_sessions', { prefix: 'typo' });
// No compile error, but crashes at runtime with "Unknown command"

After (type-safe):

const result = await appInvoke('list_agent_sessions', { prefix: 'test' });
// Type error if command name or args are wrong
// Return type is { id: string; title: string; ... }[] automatically

Benefits

• ✅ Eliminates entire class of runtime errors from incorrect API usage
• ✅ Better IDE autocomplete and refactoring support
• ✅ Catches typos and API misuse at compile time
• ✅ Self-documenting: command structure is defined in one place

Test Plan

• ✅ 186 Rust unit tests pass
• ✅ TypeScript type check passes (all previous type errors resolved)
• Manual testing: All appInvoke calls now have proper type inference

Breaking Changes

None for runtime behavior. Type-level only: code that was previously using incorrect command names or arguments will now show TypeScript errors (which is the intended improvement).