feature: source file load from subtree

[Azhovan]

What

Add Root support to sourcefile.Options so file sources can load from a dot-path subtree (for example, hub.msa) and flatten keys relative to that subtree.

Also add typed root errors:

• sourcefile.ErrRootNotFound
• sourcefile.ErrRootNotMap
• sourcefile.ErrInvalidRoot

Plus tests and docs updates for the new behavior.

Why

Many deployments keep multiple component configs in one file. Without Root, each component must build a custom source/transformer to extract its section, which duplicates logic that sourcefile should handle directly.

Type

• Fix
• Feature
• Docs
• Performance
• Breaking change

Testing

# Commands you ran
gofmt -w sourcefile/file.go sourcefile/file_test.go
go test ./sourcefile
go test ./...
make ci

closes #47

[Copilot]

Pull request overview

Adds Root support to sourcefile so consumers can load configuration from a dot-path subtree (e.g., hub.msa) and flatten keys relative to that subtree, with typed/inspectable errors for root resolution failures.

Changes:

• Extend sourcefile.Options with Root and implement subtree resolution + new typed root errors.
• Add tests covering successful subtree loading, typed root error cases, and missing-file behavior with Root.
• Update docs to describe Root behavior and new error types; expand .gitignore entries for Go/editor artifacts.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
sourcefile/file.go	Implements Options.Root, adds root-specific sentinel errors, and resolves/validates the requested subtree before flattening.
sourcefile/file_test.go	Adds coverage for subtree loading and root error conditions, plus missing-file behavior when Root is set.
docs/configuration-sources.md	Documents Root usage and the new typed root errors.
.gitignore	Broadens ignored patterns for Go build/coverage outputs and common editor/OS artifacts.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

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