Optimize shell startup and add Codespaces support

[Copilot]

Shell startup was slow due to repeated brew --prefix calls (~100ms each), and the dotfiles lacked proper Codespaces configuration.

Performance

• Cache brew --prefix in BREW_PREFIX variable across .prompt, .completion, .autojump
• Enable DISABLE_UNTRACKED_FILES_DIRTY in oh-my-zsh for faster git status in large repos

Codespaces Support

• Add .devcontainer/devcontainer.json with Ubuntu base, post-create install, and zsh default
• Skip macOS-specific plugins (dash, macos) when CODESPACES env var is set
• Skip macOS-specific runcom files in Codespaces

# Before: multiple slow calls
ZSH_SYNTAX_HIGHLIGHTING_FILE=$(brew --prefix)/share/zsh-syntax-highlighting/...
ZSH_AUTOSUGGEST_FILE=$(brew --prefix)/share/zsh-autosuggestions/...

# After: single cached call
BREW_PREFIX=${BREW_PREFIX:-$(brew --prefix)}
ZSH_SYNTAX_HIGHLIGHTING_FILE=$BREW_PREFIX/share/zsh-syntax-highlighting/...

CI/Dockerfile

• Upgrade actions/checkout v2 → v4
• Fix node unlinking for newer macOS runners
• Fix Dockerfile user creation (UID/GID conflicts with ubuntu image)
• Use JSON syntax for CMD
• Skip Homebrew casks in macOS CI via HOMEBREW_BUNDLE_CASK_SKIP

Misc

• Fix typo: ZSH_SYNTAX_HIGHLITING_FILE → ZSH_SYNTAX_HIGHLIGHTING_FILE

Original prompt

Improve my dotfiles for performance and for use in a GitHub Codespace

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[lumaxis]

@copilot Skip Homebrew casks in macOS CI, similar to #9

[Copilot]

@copilot Skip Homebrew casks in macOS CI, similar to #9

Added in 1876189. The CI workflow now sets HOMEBREW_BUNDLE_CASK_SKIP to skip all casks defined in the Brewfile during macOS CI runs.