Fix zero-result query exit codes

[Widthdom]

Summary

• Make valid zero-result query commands exit 0 by default.
• Add --strict-not-found so scripts can opt back into exit code 2 for zero-row results.
• Keep missing indexed paths, such as outline for a non-indexed file, returning exit code 2.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RunSearch_StrictNotFoundReturnsNotFoundForZeroResults_Issue1425|FullyQualifiedName~RunInspect_StrictNotFoundReturnsNotFoundForEmptyAnalysis_Issue1425" -p:UseSharedCompilation=false --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Note: a targeted run including RunPublishedTrimmedCli_SerializesQueryJsonAndErrorJson passed the new tests but hit an existing net8 ILLink AccessViolationException during trimmed publish; net9 completed that selection successfully.

Docs and Changelog

• Updated README.md, USER_GUIDE.md, and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/1425.changed.md.

Fixes #1425