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.

knip.json

@@ -37,6 +37,10 @@
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]
     },
+    "packages/web/web-search-deepseek": {
+      "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
+      "project": ["src/**/*.ts", "tests/**/*.ts"]
+    },
     "packages/ui/acp-agent": {
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]