Add GitHub Actions CI for dotfiles installation

[akan72]

Summary

• Add GitHub Actions installation checks for macOS 26 arm64 and Ubuntu 24.04 x64.
• Add clean, non-root Ubuntu and Amazon Linux container checks, idempotency coverage, security linting, and Dependabot.
• Stack position 3 of 15, after Make assimilate.sh cross-platform (macOS + Linux) #12; this PR remains a draft.

Test Plan

• Ran the complete installer twice in a fresh synthetic home on the Mac mini; backup churn changed from 15 entries before this PR to 0 after.
• Verified CI resolves symlink targets canonically and supports both Packer and lazy.nvim layouts across later stack layers.
• Ran scripts/ci/test-vps-container.sh, bash -n assimilate.sh bashrc bash_profile claude/statusline.sh scripts/ci/*.sh, zsh -n zshrc, and git diff --check.
• GitHub Actions run 32594719525 passed all eight jobs after the stack-aware CI fixes.
• Screenshots: not applicable; validation was CLI-based.