Draft: add Fog substrate positioning note

[mdheller]

Summary

This PR captures the SourceOS-side substrate role for the Fog layer.

It adds docs/FOG_SUBSTRATE.md to make the OS-layer responsibility explicit:

• local storage substrate prerequisites
• canonical /srv/fog/* directory contract
• container-host baseline expectations
• TopoLVM readiness as substrate/infrastructure, not distributed semantics

Why this belongs here

SourceOS is the immutable local-first substrate for workstation + edge lanes. The Fog layer depends on this repo for machine invariants, not for topic replication logic or compute marketplace semantics.

Intentionally not included

This PR does not add:

• ignition units
• disk bootstrap scripts
• container runtime configs
• distributed topic semantics
• compute settlement logic

Those belong in downstream repos after the contract surface is stabilized.

Follow-up

Subsequent PRs should land:

1. substrate-level mount and storage conventions
2. first-boot realization in socios-ignition
3. workstation/CI conformance checks in workstation-contracts

[mdheller]

@copilot fix failed checks

[Copilot]

Pull request overview

Adds documentation clarifying the OS-substrate responsibilities that SourceOS provides to support the Fog layer, focusing on stable local storage + filesystem contracts and clear layering boundaries.

Changes:

• Introduces a new doc describing what Fog-related substrate concerns belong in SourceOS (and what explicitly does not).
• Defines a canonical /srv/fog/* directory contract and expectations for container-host posture.
• Documents TopoLVM readiness as an infrastructure prerequisite rather than distributed semantics.

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[Copilot]

The list under SourceOS should define the default local execution posture...: starts without a blank line. With markdownlint enabled in CI, this will trigger MD032. Insert a blank line before the list items.

[Copilot]

The list after Important layering rule: is missing a blank line before the list items. This is flagged by markdownlint (MD032) and can fail CI. Add a blank line between the label line and the list.

Suggested change

	When a node participates in Kubernetes lanes, the substrate should provide the prerequisites needed for a local CSI/LVM stack such as TopoLVM.
	Important layering rule:

[Copilot]

The list starting immediately after This includes: is not separated by a blank line. This violates markdownlint's default MD032 (lists should be surrounded by blank lines) and will fail the CI markdownlint step. Add a blank line between the paragraph and the list.

[Copilot]

@copilot fix failed checks

Fixed in commit 35f348f. I resolved the failing CI checks by fixing markdownlint MD032 spacing violations in docs/FOG_SUBSTRATE.md; this is a docs-only change with no UI impact, so no screenshot applies.