docs: include_usage streaming semantics changed; dsh docs major refresh with new wire-extension reference

[github-actions]

Two developer-facing changes landed in this fetch. For DeepSeek API consumers, the documented include_usage streaming behavior changed in both Chat Completions and the FIM beta: there is no longer a separate usage-only chunk before data: [DONE]. Token statistics now ride on the final content chunk, whose choices array holds exactly one element with no new content and a non-null finish_reason — a wire-shape change that can break stream parsers that keyed off the old empty-choices usage chunk. Separately, the dsh (DeepSeek Harness) developer-preview docs saw a major refresh: eight new pages, one deleted tutorial, a brand-new reference for DeepSeek's official LLM API wire extensions, and regenerated module/event/config catalogs.

• include_usage reworked in api/create-chat-completion.md and api/create-completion.md (en + zh-cn): usage is null on every stream chunk except the last when set to true; omitted/false removes the field from all chunks except the last. No separate usage-only chunk is emitted anymore.
• guides/thinking_mode.md (en + zh-cn): whether reasoning_content must be passed back and is concatenated into context now depends on the tools parameter — with tools it must be echoed back (else 400), without tools it is ignored.
• dsh: new deepseek-llm-api-wire-extensions.md documents the dsh_plugin_packages and dsh_session_log request-body extensions, x-deepseek-harness-* headers, and the DeepSeekLlmApiExtensionRegistry registration model.
• dsh: cookbook adding-a-conversation-node.md deleted; new subsystem pages cover conversation assembly, Web Client architecture + slots, webhook runtime, and a Todo subsystem; new guides cover GitHub-webhook review sessions, third-party MCP memory servers, and session-local scheduling, plus a dynamic-Cordis practice page.
• dsh: config-catalog.md adds plugin sections (api-gateway, api-session-controller, api-settings-controller, experimental-inspector, plugin-package-inventory-deepseek, sdk-app, session-log-deepseek, session-log-export), and module-graph.md / event-producer-consumer.md were regenerated with new agent/* and api-session/* events. No npm version bump in this batch (dsh/npm.md unchanged); dsh-doc-standards skill renamed to dsh-doc.