docs: clarify repo topology and connected surfaces

[mdheller]

Summary

Clarifies that SourceOS is the immutable substrate in the Linux-side topology and names the adjacent integration, contract, automation, and public-surface repos.

Why

This repo is central to the stack, but the README did not yet say how it fits with the spine, contract, and commons lanes. This update makes those boundaries explicit.

[Copilot]

Pull request overview

Updates the README to clarify where this repository sits in the broader SourceOS/SociOS Linux-side topology, explicitly describing its role as the immutable OS substrate and naming adjacent integration/contract/automation/public-surface repositories.

Changes:

• Added a “Topology position” section describing this repo’s role in the stack.
• Listed connected repositories/surfaces and clarified what is explicitly out of scope for this repo.
• Documented intended semantic direction for publishing a substrate-focused repo descriptor referencing sourceos-spec.

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[mdheller]

@copilot fix failed checks and conflicts

[Copilot]

@copilot fix failed checks and conflicts

Fixed in bb2adda. The CI was failing due to yamllint errors in spec/volumes/volumes.yml — alignment spaces after colons and commas (e.g. Desktop: {path: "~/Desktop", policy:) violated the [colons] and [commas] rules. Removed the extra alignment padding. No merge conflicts were found in the branch.