First image garden implementation (Infra)

[fernando79513]

Description

Added the first image garden pipelines into CI.
This PR introduces the first image-garden based workflows.
The workflows will be run after finishing the native snap builds, on demand or when any changes have been introduced to the image garden workflows.
The current tested snaps are:

• ubuntu-core-18 (amd64)
• ubuntu-core-20 (amd64)
• ubuntu-core-22 (amd64)
• ubuntu-core-24 (amd64)
• ubuntu-core-26 (amd64)

In the follow-up PRs we will deal with:

• Using the new smoke test plan
• Local patching and execution of snaps with image garden
• Dispatch patched snaps in the lab

Resolved issues

CHECKBOX-2270

Documentation

Tests

The CI pipelines should be enough to test this cange. Once this is in place, we will see if its executed poperly after the native builds step

It's already possible to test this locally, but I think I will create a python tool arround it, since we will probably need it to patch the snap easily:

CHECKBOX_CHANNEL=edge image-garden.spread -vv -artifacts=artifacts garden:ubuntu-core-20:tests/run-smoke

[Copilot]

🟡 Changes recommended

The spread suite configuration is currently inconsistent with the configured SSH user (likely breaking the run) and the smoke task installs snaps in devmode, reducing the validity of “store snap” validation.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds an initial “image-garden + spread” smoke validation pipeline to the Checkbox GitHub Actions CI, intended to exercise freshly published store snaps on Ubuntu Core VMs as part of daily builds.

Changes:

• Introduces a reusable workflow to run spread smoke tests via zyga/image-garden-action across multiple Ubuntu Core versions.
• Hooks the new smoke workflow into the existing daily builds pipeline (edge channel).
• Adds a dedicated spread suite (.github/Image-garden-spread/) including spread configuration, a smoke task, and a launcher config.

File summaries

File	Description
.github/workflows/smoke-spread-store.yaml	New reusable workflow to run spread smoke suite via image-garden across UC18/20/22/24.
.github/workflows/daily-builds.yml	Adds a daily job to run the new smoke workflow after snap uploads.
.github/Image-garden-spread/tests/run-smoke/task.yaml	Adds the smoke task that downloads and installs the matching Checkbox snap and runs the launcher.
.github/Image-garden-spread/tests/run-smoke/launcher.conf	Launcher config to run com.canonical.certification::smoke and emit local reports/artifacts.
.github/Image-garden-spread/spread.yaml	Spread backend/system definitions for image-garden-based allocation/discard and suite discovery.
.github/Image-garden-spread/.gitignore	Ignores downloaded snaps/assertions and local image-garden/spread state/artifacts.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Lite

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