Simplify cdidx code-search policy

[Widthdom]

Summary

• replace the duplicated, version-specific cdidx setup, freshness, query, fallback, SQL, and command examples in AGENT_GUIDE.md with a concise policy
• direct agents to cdidx's own MCP instructions/tool descriptions or CLI help/diagnostics
• identify Widthdom/CodeIndex as the official source without duplicating installation commands
• keep the English and Japanese guidance aligned and record the documentation change under [Unreleased]

Why

cdidx now exposes current usage guidance through its supported MCP and CLI surfaces. Keeping detailed command recipes in this repository duplicated that guidance and allowed the agent policy to drift as cdidx evolved.

Impact

Agent instructions are shorter and version-independent. Agents still have an explicit cdidx-first requirement, an official source link, and a prohibition on shell search/discovery commands for repository search.

Validation

• dotnet test FolderDiffIL4DotNet.Tests/FolderDiffIL4DotNet.Tests.csproj --configuration Release --nologo — 2,044 passed, 1 skipped, 0 failed
• cdidx status --check --json — index matches workspace
• cdidx validate --path AGENT_GUIDE.md --json=array — no issues
• cdidx validate --path CHANGELOG.md --json=array — no issues
• git diff --check — clean

Notes

The commit is intentionally unsigned at the maintainer's request after the configured SSH signing key required interactive passphrase entry in the automation environment.