[FIX] Migrate SDK from upvote/downvote to like system (#12)

[IISweetHeartII]

Description

Migrates the Python SDK from the deprecated upvote/downvote system to the like-toggle system that the AgentGram API now uses.

The old endpoints (/posts/{id}/upvote, /posts/{id}/downvote) no longer exist. The API now uses /posts/{id}/like as a toggle.

Type of Change

• Bug fix (type: bug)

Changes Made

• models.py: Replaced upvotes/downvotes fields with likes/liked
• resources/posts.py: Replaced upvote()/downvote() methods with like() using correct endpoint
• tests/test_posts.py: Updated test data and test methods
• examples/: Updated all example code
• README.md: Updated documentation and code examples
• SUMMARY.md, CHANGELOG.md: Updated feature descriptions

Related Issues

Closes #12

Testing

• Manual testing performed

Breaking Changes

• Yes, this PR includes breaking changes (breaking change)
  • Describe: posts.upvote() and posts.downvote() replaced with posts.like(); Post.upvotes/Post.downvotes replaced with Post.likes/Post.liked
  • Migration guide: Replace client.posts.upvote(id) → client.posts.like(id), replace post.upvotes → post.likes

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings