feat(web): add DeepSeek-backed web search provider

Add @deepseek-ai/dsh-web-search-deepseek: a WebSearchProvider that calls
DeepSeek's Anthropic-compatible Messages API with the native
web_search_20250305 server tool and parses the structured
web_search_tool_result blocks into the ctx.web seam's WebSearchResult.

- Namespace plugin (inject: ['web']), no default export — registers into
  ctx.web like dsh-llm-deepseek registers into ctx.llm.
- Strict mode: a response with no web_search_tool_result block throws
  WEB_PROVIDER_ERROR rather than scraping URLs from model prose.
- Reuses $DEEPSEEK_API_KEY; baseURL defaults to the Anthropic-compatible
  base (api.deepseek.com/anthropic/v1) and does NOT reuse
  $DEEPSEEK_BASE_URL, which belongs to the chat-completions LLM adapter.
- snippet joined from text-block citations; sources deduped by url.
- Two-stage build layout (outDir lib/types) matching the other web
  packages; registered in tsconfig.json, tsconfig.build.json, knip.json,
  and docs/module-graph.md.

docs/module-graph.md

@@ -25,6 +25,7 @@ graph TD
   llm-replay --> session
   session-persistence --> session
   web-fetch-local --> web
+  web-search-deepseek --> web
   web-search-exa --> web
   web-search-perplexity --> web
   invariants --> agent
@@ -116,6 +117,7 @@ graph TD
 | `llm-replay` | `llm`, `session` |
 | `session-persistence` | `session` |
 | `web-fetch-local` | `web` |
+| `web-search-deepseek` | `web` |
 | `web-search-exa` | `web` |
 | `web-search-perplexity` | `web` |
 | `invariants` | `agent`, `llm`, `session` |