Add YARD documentation across Minisky classes

[mackuba]

Motivation

• Provide YARD-style documentation for library types to improve code discoverability and developer ergonomics.
• Clarify behavior and parameters for the client lifecycle, HTTP helpers, and token management.
• Document error types and the public VERSION constant so generated docs accurately describe runtime semantics.

Description

• Added YARD comments to Minisky in lib/minisky/minisky.rb (class, initialize, active_repl?, save_config).
• Added YARD comments throughout lib/minisky/requests.rb including Minisky::User, NSID_REGEXP, the Requests module and public methods (get_request, post_request, fetch_all, check_access, log_in, perform_token_refresh, token_expiration_date, access_token_expired?, reset_tokens) and private helpers (make_request, build_request_uri, authentication_header, handle_response), plus Ruby 2 compatibility wrappers.
• Documented error classes in lib/minisky/errors.rb with parameter and return annotations for BadResponse, UnexpectedRedirect, and FieldNotSetError.
• Annotated VERSION in lib/minisky/version.rb and added small doc comments to the spec helper spec/custom_client_spec.rb for the test client.

Testing

• No automated tests were executed as part of this change.
• Changes are purely documentation and spec-comment updates and do not alter runtime logic according to the diff.

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Codex Task