refactor: replace authentication field with envVars in TPMJS specification

Replace the authentication field with a more general envVars array that allows tools to specify required environment variables:

Changes to type definitions:
- Remove TpmjsAuthenticationSchema and TpmjsAuthentication type
- Add TpmjsEnvVarSchema with fields: name, description, required, default
- Replace authentication field with envVars array in TpmjsRichSchema
- Update validateTpmjsField to check envVars instead of authentication

Changes to documentation:
- Update /spec page to document envVars instead of authentication
- Update /publish page examples to use envVars
- Update HOW_TO_PUBLISH_A_TOOL.md with envVars examples
- Update validation errors section
- Remove authentication from @tpmjs/createblogpost example

The envVars field is more flexible and clearer - it lists all environment variables a tool needs (API keys, endpoints, config values) rather than trying to categorize authentication types.

Example:
```json
"envVars": [
  {
    "name": "OPENAI_API_KEY",
    "description": "API key for OpenAI services",
    "required": true
  }
]
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/tools/createBlogPost/package.json

@@ -80,10 +80,6 @@
       "type": "BlogPost",
       "description": "A structured blog post object with frontmatter, content, and metadata including slug, wordCount, readingTime, and formattedOutput"
     },
-    "authentication": {
-      "required": false,
-      "type": "api-key"
-    },
     "frameworks": ["vercel-ai", "langchain"],
     "links": {
       "documentation": "https://tpmjs.com/tools/createblogpost",