Merge syntax-highlighting branch: add syntax highlighting and intra-line diff

- Syntax highlighting for markdown code blocks, read/write tool output
- Intra-line diff highlighting for edit tool with word-level inverse
- VS Code-style syntax colors in themes
- Fix Google provider FinishReason handling
- Upgrade @google/genai to 1.34.0

AGENTS.md

@@ -15,6 +15,8 @@ read README.md, then ask which module(s) to work on. Based on the answer, read t
 - No `any` types unless absolutely necessary
 - Check node_modules for external API type definitions instead of guessing
 - No inline imports like `await import("./foo.js")`
+- NEVER remove or downgrade code to fix type errors from outdated dependencies; upgrade the dependency instead
+- Always ask before removing functionality or code that appears to be intentional
 
 ## Commands
 - After code changes: `npm run check` (get full output, no tail)