Add MCP health and keep-alive probes

[Widthdom]

Summary

• Add structured MCP ping health data and HTTP GET /healthz on the existing MCP HTTP listener.
• Add opt-in HTTP /events keep-alive notifications via CDIDX_MCP_KEEP_ALIVE_INTERVAL_S.
• Document the MCP health/keep-alive contracts and add bilingual changelog fragments.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName=CodeIndex.Tests.McpServerTests.ProcessLineAsync_PingReturnsStructuredHealth|FullyQualifiedName=CodeIndex.Tests.HttpMcpTransportTests.HttpTransport_Healthz_ReturnsStructuredHealth|FullyQualifiedName=CodeIndex.Tests.HttpMcpTransportTests.HttpTransport_EventsStream_EmitsOptInKeepAliveNotifications" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• Adversarial review: No blocking/actionable issues found.

Documentation and changelog

• README.md
• DEVELOPER_GUIDE.md
• changelog.d/unreleased/1740.added.md
• changelog.d/unreleased/1791.added.md

Notes

• A broader McpServerTests|HttpMcpTransportTests run was blocked by existing local git signing configuration in tests that create commits (id_ed25519_signing passphrase prompt), unrelated to this PR.
• Local cdidx status --check --json remains stale because repeated index refresh attempts ended with E012_INTERRUPTED in this environment.

Fixes #1740
Fixes #1791