Add human-readable error messages for missing token file

[konard]

Summary

• Replace raw ENOENT errors with helpful, human-readable messages for missing token files
• Add specific guidance on how to create and configure the token file
• Improve error handling across all file reading operations in the codebase
• Add comprehensive error handling for JSON parsing failures

Changes Made

• utils.js: Enhanced getToken() with detailed error message including setup instructions and VK API documentation link
• utils.js: Improved readTextSync() and readJsonSync() with better error messages for missing files and invalid JSON
• filter-stickers.js: Added error handling for missing/invalid received-attachments.json
• convert-messages-to-markdown.js: Added JSON parsing error handling
• triggers/attachments.js: Added graceful error handling that allows continuation with empty state

Before (Issue #33)

Error: ENOENT: no such file or directory, open 'token'
    at Object.openSync (node:fs:601:3)
    at Object.readFileSync (node:fs:469:35)

After

Error: Token file "token" not found. Please create a file named "token" containing either:
  1. Your VK API access token (a string of characters)
  2. A VK OAuth URL with access_token parameter

For more information on how to get a VK API token, visit: https://vk.com/dev/access_token

Test Plan

• Verified all scripts using getToken() show improved error messages
• Tested file reading functions with missing files
• Tested JSON parsing with invalid JSON files
• Created comprehensive test suite in experiments/ folder
• Confirmed existing functionality works with valid token files

🤖 Generated with Claude Code

[konard]

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