ci: add Snapshot Build workflow (parallel matrix)

[chaodu-agent]

Summary

Adds a workflow_dispatch workflow to build any branch × all variants in parallel without bumping the beta tag.

Inputs

Input	Default	Description
branch	main	Branch to build
variants	all	Comma-separated list or all for full matrix
tag	snapshot-<sha>	Custom image tag (optional)

Usage

# Build ALL 14 variants from main in parallel
gh workflow run snapshot-build.yml -f branch=main -f tag=pre-beta

# Build specific variants only
gh workflow run snapshot-build.yml -f branch=main -f variants=kiro,claude,codex -f tag=pre-beta

# Single variant
gh workflow run snapshot-build.yml -f branch=main -f variants=claude -f tag=test

Architecture

• 14 variants × 2 platforms (amd64 + arm64) = up to 28 parallel build jobs
• Per-variant manifest merge produces multi-arch images
• fail-fast: false — one variant failure doesn't cancel others
• GHA cache scoped per variant + platform

Why

Need a way to build images from main for pre-beta testing without promoting to beta. PR Preview requires a PR number; Release PR bumps the version. This fills the gap.