[codex] Fix index watch failure handling

[Widthdom]

Summary

• Propagate index --watch sub-run failures by emitting failed watch batch events with exit_code, adding exit codes to human summaries, and returning the latest non-zero sub-run exit code when watch mode stops.
• Cap pending watch path batches and collapse excessive unique paths into a full incremental rescan instead of retaining an unbounded path set.

Documentation / Changelog

• Updated USER_GUIDE.md watch-mode behavior in English and Japanese.
• Added changelog fragments:
  • changelog.d/unreleased/2902.fixed.md
  • changelog.d/unreleased/2903.fixed.md

Validation

• dotnet build
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexWatchRunnerTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net9.0 --filter "FullyQualifiedName~InvokeSubRunAndEmit"
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Full-suite dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore was attempted; unrelated ambient workspace failures were filed as follow-up #2951.

Issues

Fixes #2902
Fixes #2903

Follow-up: #2951