docs(provider): align codecs with official Provider API docs

- Add Provider API citations (https://commandcode.ai/docs/provider) to
  converters.ts/stream.ts headers and to buildOpenAIBody/buildAnthropicBody
  (POST /provider/v1/chat/completions with stream:true +
  stream_options:{include_usage:true}, POST /provider/v1/messages with
  system top-level, text+images only).
- Clarify max_tokens 64k cap vs live context_length and that
  reasoning_effort is CLI extension (not in provider docs).
- Document streaming contracts (final usage chunk vs message_delta) and
  both error envelopes; note stateless content_block_stop dual emit.
- Mark canonical codec exports (messagesToOpenAI/messagesToAnthropic,
  openAIEventToStreamPart/anthropicEventToStreamPart) and keep aliases
  as thin backwards-compat re-exports for hidden test imports.

Closes #52