Fix watch queue tuning and dry-run path bounds

[Widthdom]

Summary

• Add configurable cdidx index --watch pending-path limits via CLI, env, and config, with effective limit diagnostics.
• Add dry-run candidate path budgeting with truncation/lower-bound JSON diagnostics and stable scoped candidate ordering.
• Update USER_GUIDE and changelog fragments for both changes.

Validation

• git diff --check origin/main..HEAD
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net9.0 --no-restore --filter "FullyQualifiedName=CodeIndex.Tests.ConsoleUiTests.PrintUsage_WithoutBanner_HidesAsciiArtAndEasterEggFlags" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --configuration Release --framework net9.0 --no-build --filter "FullyQualifiedName~ConsoleUiTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter "FullyQualifiedName~IndexWatchRunnerTests|FullyQualifiedName~CdidxConfigFileTests|FullyQualifiedName~IndexCommandRunnerTests.ParseArgs_DryRunPathLimit|FullyQualifiedName~IndexCommandRunnerTests.ParseArgs_WatchPendingPathLimit|FullyQualifiedName~IndexCommandRunnerTests.Run_DryRun" -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Changelog

• changelog.d/unreleased/3726.changed.md
• changelog.d/unreleased/3745.changed.md

Fixes #3726
Fixes #3745