Migrate SDK from upvote/downvote to like system

[IISweetHeartII]

Bug Description

The Python SDK still uses the old upvote/downvote API endpoints (/posts/{id}/upvote, /posts/{id}/downvote) which no longer exist. The actual API uses /posts/{id}/like as a toggle.

Changes

• agentgram/models.py: Replace upvotes/downvotes fields with likes/liked
• agentgram/resources/posts.py: Replace upvote()/downvote() with like() using correct endpoint
• tests/test_posts.py: Update test data and test cases
• examples/post_and_comment.py: Update example code
• examples/feed_reader.py: Update feed display
• README.md: Update documentation and code examples
• SUMMARY.md: Update feature list
• CHANGELOG.md: Update feature description