runtime: add sourceos-shell search-provider scaffold and invariant config

[mdheller]

Summary

Stacked on top of the current sourceos-shell Linux realization chain, this PR adds the first explicit search-provider scaffold for the temporary launcher bridge and records the no_redundant_file_search invariant in both config and tests.

Added files

• linux/desktop/sourceos-search-provider.conf

Updated files

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

What it does

• adds sourceos.shell.searchProvider.mode
• adds sourceos.shell.searchProvider.linuxFileProvider
• realizes /etc/sourceos-shell/search-provider.json
• records the rollout invariant:
  • apps -> launcher
  • files -> Linux-native-only
  • web -> browser-agent
• extends the shell module contract test to assert the search-provider scaffold exists and records no_redundant_file_search

Why

The previous Linux PRs established service scaffolds and a launcher bridge note, but they did not yet materialize the search-provider policy as a concrete Linux-facing artifact. This PR closes that gap while the runtime repo still does not exist.

Stack relationship

This PR is stacked on top of:

• source-os#26
• source-os#70
• source-os#71

Follow-on

• replace scaffold config with real runtime-owned config once SourceOS-Linux/sourceos-shell exists
• bind the launcher bridge implementation to this config surface
• upgrade tests from scaffold assertions to actual routing/service checks