Add Chat module for reading publication subscriber chats

[danwagnerco]

Introduces Chat, ChatThread, and ChatMessage classes that provide read access to Substack community chat threads and messages. Requires authentication via SubstackAuth.

Features:

• Chat.get_threads() to list threads in a publication chat
• ChatThread.get_messages() to read replies within a thread
• Custom exceptions: ChatAuthenticationRequired, ChatAccessDenied, ChatNotFound, ThreadNotFound

Includes API reference docs, user guide updates, and full test coverage with self-contained fixtures.

[NHagar]

Thanks for this contribution! The code is clean and well-structured — the Chat → ChatThread → ChatMessage hierarchy fits nicely with the existing patterns in the library, and the test coverage and documentation are thorough.

A few things I'd like to see addressed before merging:

1. Unrelated changes should be split out. The .gitattributes file, the .gitignore additions (site/, .claude/, .devcontainer/), and the uv.lock churn (just upload-time metadata additions, no actual dependency changes) add noise to the PR. Could you remove these or submit them separately?
2. Dead fixture paths in tests. The test fixtures try to load JSON from .claude/v1_community_publications_*.json, but that path is gitignored and not included in the PR. The fallback data works fine, so I'd suggest just removing the file-loading code path to avoid confusion.
3. Client-side limit is misleading. get_threads(limit=5) fetches all threads from the API and slices locally. Same for get_messages(). This should either pass limit through to the API as a query param, or be clearly documented as client-side truncation of a single page.
4. publication_id usability. The rest of the library works with subdomain strings, but Chat() requires a numeric publication ID. Would be great to accept a subdomain and resolve it internally, or at minimum document how users can find their publication ID.

[NHagar]

Closing in favor of #21