Out-of-tree --config is rejected by the scoped filesystem and reported as a missing file

[Mpdreamz]

Summary

Commands that accept --config pointing at a changelog.yml outside the current working tree fail, because the ScopedFileSystem they read through does not include the config file's own directory as a scope root.

Split out of #3742, where this surfaced. It is not a regression from that PR — the same limitation exists on main.

Detail

ChangelogPrEvaluationService reads config through the ScopedFileSystem it is handed:

private readonly ChangelogConfigurationLoader _configLoader =
    new(logFactory, configurationContext, fileSystem ?? FileSystemFactory.RealRead);

• On main this defaults to FileSystemFactory.RealRead — scoped to Paths.WorkingDirectoryRoot + Paths.ApplicationData.
• After Make PR_BODY_FILE work in CI by scoping reads to RUNNER_TEMP #3742, evaluate-pr passes FileSystemFactory.RealReadForCI, which is the same scope plus RUNNER_TEMP.

Neither includes the directory the --config path actually lives in, so a config outside the working tree is out of scope.

Why it is confusing

ScopedFileInfo.Exists returns false for an out-of-scope path rather than throwing. The failure therefore presents as "the file does not exist" even when the file is plainly there on disk, which sends you looking for a typo'd path instead of a scope boundary.

This affects any caller that surfaces existence before reading, not just the changelog commands.

Repro sketch

docs-builder changelog evaluate-pr --config /some/path/outside/repo/changelog.yml ...
# reports the config as missing; the file exists and is readable

Possible directions

1. Add the resolved config file's directory as an explicit scope root when a config path is supplied — a short-lived branch in Make PR_BODY_FILE work in CI by scoping reads to RUNNER_TEMP #3742 did this via a ChangelogPrEvaluationFileSystem helper, dropped in favour of the simpler RealReadForCI factory method.
2. Fold it into FileSystemFactory so every --config-accepting command benefits rather than one command at a time.
3. Independently: make an out-of-scope read distinguishable from a genuinely missing file, so the diagnostic points at the scope rather than at the path.

Worth deciding whether out-of-tree --config is a supported scenario at all before implementing — for CI-only commands the config is always in-repo, so option 3 alone may be sufficient.