Add guard-aware search filters

[Widthdom]

Summary

• Added guard-aware CLI/MCP search filters for before/after require/reject constraints.
• Emit guard evidence for required guards and keep count/search behavior aligned.
• Fixed guarded MCP pagination and tightened non-exact multi-token guard focus lines.

Validation

• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~Issue2852
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter FullyQualifiedName~Issue2852 -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Note: a full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false run was started earlier but did not produce a final summary after about 21 minutes and was stopped; the focused Debug/Release Issue2852 suites passed.

Fixes #2852