Add Linux realization surfaces for sourceos-shell services and desktop integration

[mdheller]

Summary

Add the Linux/Nix realization surfaces needed to run the planned SourceOS-Linux/sourceos-shell product on Linux hosts and images.

This issue covers only Linux realization. It should not redefine shared schemas or control-plane semantics locally.

Why this belongs here

source-os is already documented as the Linux realization home for SourceOS control-plane work. The shell/services need Nix modules, systemd units, desktop integration, and profile wiring here.

Scope

1. Nix/NixOS modules

Add first-cut modules for:

• sourceos-shell
• sourceos-router
• sourceos-pdf-secure
• optional sourceos-docd

2. Desktop integration

Add Linux-facing assets for:

• desktop entry / launcher wiring
• command-palette/search provider integration
• user-session startup hooks
• XDG/config paths

3. Systemd user services

Add user or host services for:

• shell-web runtime
• router service
• pdf-secure service
• docd watcher (optional in first cut)

4. Profiles / tests

Wire the shell into at least one dev profile and add a minimal integration test proving:

• services start
• shell can reach router and pdf-secure
• desktop entry exists

Explicit non-scope

Do not add shared schema canon here. Those definitions belong in SourceOS-Linux/sourceos-spec.

Acceptance criteria

• Nix modules exist for shell/router/pdf-secure (and optionally docd)
• Desktop entry and startup wiring exist in the repo
• At least one profile includes the shell stack
• A minimal test verifies the service graph and desktop integration surface
• Documentation links back to the control-plane/spec repos instead of redefining semantics locally

Follow-on dependency

This issue assumes a new product repo SourceOS-Linux/sourceos-shell will carry the actual application/runtime code.

Proposed owner

@mdheller