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.

packages/web/web-search-deepseek/package.json

@@ -0,0 +1,35 @@
+{
+  "name": "@deepseek-ai/dsh-web-search-deepseek",
+  "description": "DeepSeek-backed search provider (native web_search via the Anthropic-compatible API) for the DeepSeek Harness web capability seam (ctx.web)",
+  "version": "0.0.1",
+  "private": true,
+  "type": "module",
+  "main": "lib/index.js",
+  "types": "lib/types/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./lib/types/index.d.ts",
+      "default": "./lib/index.js"
+    },
+    "./src/*": "./src/*",
+    "./package.json": "./package.json"
+  },
+  "files": [
+    "lib/index.js",
+    "lib/types/**/*.d.ts",
+    "lib/types/**/*.d.ts.map",
+    "src"
+  ],
+  "license": "BSD-3-Clause",
+  "peerDependencies": {
+    "@deepseek-ai/dsh-web": "^0.0.1",
+    "cordis": "^4.0.0-rc.6"
+  },
+  "dependencies": {
+    "schemastery": "^3.18.0"
+  },
+  "devDependencies": {
+    "@deepseek-ai/dsh-web": "workspace:^",
+    "cordis": "^4.0.0-rc.6"
+  }
+}