runtime: add sourceos-shell contract test and launcher bridge note

[mdheller]

Summary

Stacked on top of the initial Linux bootstrap for sourceos-shell, this PR adds the first contract-style test scaffold, wires the linux-dev profile to import the shell profile scaffold, adds a flake check, and documents the temporary launcher bridge rule.

Added files

• tests/sourceos-shell-module-contract.nix
• docs/sourceos-shell-launcher-bridge.md

Updated files

• profiles/linux-dev/default.nix
• flake.nix

Why

The initial Linux bootstrap PR adds module/profile/desktop/service scaffolds. This follow-on PR starts enforcing those surfaces with a test and records the launcher-routing invariant:

• apps -> launcher provider
• files -> Linux-native file search only
• web -> browser/web agent

with no redundant file search for files queries.

Stack relationship

This PR is stacked on top of:

• source-os#26

Follow-on

• replace placeholder service commands with real shell/router/pdf-secure packages
• add additional integration tests for service graph and desktop entry installation
• land Albert bridge code or patches tracked in issue #16