Fix self-improvement search policy references

[Widthdom]

Summary

• Replace stale SELF_IMPROVEMENT.md references to removed README headings with the authoritative AGENT_GUIDE.md Search and Indexing Rules.
• Keep the English and Japanese hard rules and commit checklists synchronized.
• Add the bilingual changelog fragment changelog.d/unreleased/4672.docs.md.

Root cause and impact

The self-improvement workflow still named README headings that no longer exist, so agents could not complete the documented precommit review unambiguously. The workflow now points to the current authoritative policy and precommit workflow.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• bash .codex/scripts/check-workflow-links.sh
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter FullyQualifiedName~DocumentationDriftTests -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• git diff --check
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• Updated SELF_IMPROVEMENT.md in English and Japanese.
• Added changelog.d/unreleased/4672.docs.md.
• No direct CHANGELOG.md changes.

Follow-up candidates

None.

Fixes #4672