F5: web search/fetch on the Kimi services (kimi-cli parity)

web_search now speaks the Kimi search service (kimi-cli tools/web/search.py,
wire-verified against api.kimi.com):
- POST {coding_base}/search with {text_query, limit 1-20 (default 5),
  enable_page_crawling, timeout_seconds: 30}, OAuth bearer +
  X-Msh-Tool-Call-Id; results render in kimi-cli's Title/Date/URL/Summary
  schema with result URLs as citations.
- The old implementation called the xAI Responses API with a search model;
  that client is fully replaced and the entire model-based config surface
  is excised root-and-branch: web_search_model config keys/env/CLI plumbing,
  resolve_web_search_sampling_config, toolset web_search SamplerConfig,
  RemoteSettings.web_search_model, default_web_search_model.
- Enablement is now purely structural: the service exists only on the Kimi
  Code subscription channel, so OAuth sessions get Enabled and API-key-only
  sessions get Disabled (tool absent) — per PRD F5.

web_fetch gains the Kimi fetch service as its primary path (kimi-cli
tools/web/fetch.py): POST {coding_base}/fetch with {url}, Accept:
text/markdown, OAuth bearer + X-Msh-Tool-Call-Id; the 200 body is the
extracted markdown (still overflow-budgeted). Any service failure falls
back to the existing local pipeline (SSRF guards, cache, extraction).
The tool gate defaults ON now (kimi-cli always offers FetchURL) and the
egress User-Agent no longer claims grok-agent/x.ai.

Verified end-to-end against the scripted mock service: a headless session
drove web_search (limit/crawling/call-id observed on the wire) then
web_fetch (Accept + call-id observed) to completion.

crates/codegen/kigi-shell/src/agent/subagent/mod.rs

@@ -206,7 +206,7 @@ pub(crate) struct SubagentSpawnContext {
     /// cross-session memory store.
     pub memory_config: Option<crate::config::MemoryConfig>,
     /// Resolved sampling config for web_search.
-    pub web_search_sampling_config: Option<kigi_sampler::SamplerConfig>,
+    pub web_search_config: kigi_tools::implementations::WebSearchConfig,
     /// Resolved config for web fetch.
     pub web_fetch_config: kigi_tools::implementations::grok_build::web_fetch::WebFetchConfig,
     /// Image generation config (parent-inherited).