fix: include Linux AppImage artifacts in latest.json

[Dimillian]

Motivation

• Ensure the release generator emits Linux updater metadata so updater clients can find linux-x86_64 and linux-aarch64 platforms instead of only the macOS entry.

Description

• Add artifacts_dir = Path("release-artifacts") and scan for *.AppImage.tar.gz with a fallback to *.AppImage using rglob.
• Add a simplified detect_arch helper to map filenames to x86_64 or aarch64 and populate platforms with linux-<arch> entries containing url and signature.
• Validate presence of AppImage artifacts and their .sig files using Path.with_suffix and fail fast with SystemExit if any are missing.
• Consolidate the platforms map and write the complete payload to release-artifacts/latest.json.

Testing

• No automated tests were run because this change only updates the CI workflow and does not affect runtime code paths.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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