Add more helpful error messages for common CLI mistakes

Make error messages more user-friendly when users make common mistakes.

**Tasks:**
- Add specific error when file doesn't exist
- Add hint when Ollama is not running (for PDF→MD)
- Add suggestion when wrong file type is provided
- Improve error formatting with colors (using chalk)

**Files to modify:**
- `src/cli.ts`
- `src/converters/pdf-to-markdown.ts`

**Good First Issue:** Simple TypeScript changes with clear requirements!

Start in src/cli.ts and src/converters/pdf-to-markdown.ts; trace the existing error paths for missing files, a stopped Ollama service, wrong file types, and formatting. Confirm each mistake produces the requested hint or suggestion and that error output uses chalk for colors.

Written by the indexing model from the issue text.