Document HTTP MCP keep-alive behavior

[Widthdom]

Summary

• Clarifies the DEVELOPER_GUIDE HTTP MCP /events wording so it matches the current opt-in keep-alive notification contract.
• Updates the Japanese developer-guide section with the same non-HTTP vs HTTP transport distinction.
• Adds the bilingual changelog fragment changelog.d/unreleased/2963.docs.md.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll search "current server does not yet emit unsolicited frames" --path DEVELOPER_GUIDE.md --exact-substring --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll search "server-origin emission is intentionally limited" --path DEVELOPER_GUIDE.md --exact-substring --json
• git diff --check
• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/2963.docs.md.

Review

• Codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

• None.

Fixes #2963