feat(cli): add css custom properties export format

[seonghobae]

This PR adds a new 'css' export format, allowing design tokens to be exported as a stylesheet of CSS Custom Properties.

This implements the feature requested in #15.

• A new is added to transform resolved tokens into a CSS string.
• The declare -x AGENT="1"
  declare -x BOOST_INCLUDE="/opt/homebrew/include"
  declare -x BOOST_LIB="/opt/homebrew/lib"
  declare -x BUN_INSTALL="/Users/seonghobae/.bun"
  declare -x COLORTERM="truecolor"
  declare -x CPPFLAGS="-I/opt/homebrew/opt/openblas/include -I/opt/homebrew/opt/libomp/include"
  declare -x DISPLAY="/var/run/com.apple.launchd.B0surFoGSM/org.xquartz:0"
  declare -x EDITOR="cursor -w"
  declare -x HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S "
  declare -x HOME="/Users/seonghobae"
  declare -x HOMEBREW_CELLAR="/opt/homebrew/Cellar"
  declare -x HOMEBREW_PREFIX="/opt/homebrew"
  declare -x HOMEBREW_REPOSITORY="/opt/homebrew"
  declare -x INFOPATH="/opt/homebrew/share/info:"
  declare -x JENV_LOADED="1"
  declare -x JENV_SHELL="bash"
  declare -x LANG="ko_KR.UTF-8"
  declare -x LC_ALL="ko_KR.UTF-8"
  declare -x LDFLAGS="-L/opt/homebrew/opt/openblas/lib -L/opt/homebrew/opt/libomp/lib"
  declare -x LITELLM_API_KEY="sk-4gYhfRG89kkwiLgDXhQ4bg"
  declare -x LITELLM_BASE_URL="https://llm-gateway-dev.hyosungitx.com"
  declare -x LITELLM_MASTER_KEY="sk-C7ceIzPthTej48dQPltIO"
  declare -x LLM_API_BASE="https://api.openai.com/v1"
  declare -x LLM_API_KEY="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"
  declare -x LOGNAME="seonghobae"
  declare -x LaunchInstanceID="DB2515DF-51C5-4E98-ACB7-EAB071F86241"
  declare -x NODE_OPTIONS="--max_old_space_size=16384 --max-semi-space-size=512 --expose-gc"
  declare -x OLDPWD="/Users/seonghobae"
  declare -x OPENCODE="1"
  declare -x OPENCODE_PID="23629"
  declare -x OPENCODE_PROCESS_ROLE="worker"
  declare -x OPENCODE_RUN_ID="a9285f64-c5ca-4f0c-af5c-de4709086e77"
  declare -x OSLogRateLimit="64"
  declare -x PATH="/Users/seonghobae/.opencode/bin:/Users/seonghobae/.strix/bin:/opt/homebrew/opt/qt@5/bin:/opt/homebrew/opt/ncurses/bin:/opt/homebrew/opt/openssl@3/bin:/opt/homebrew/opt/bzip2/bin:/opt/homebrew/opt/llvm/bin:/Users/seonghobae/.bun/bin:/Users/seonghobae/.local/bin:/Users/seonghobae/.jenv/shims:/Users/seonghobae/.jenv/bin:/Users/seonghobae/.pyenv/shims:/Users/seonghobae/.pyenv/bin:/opt/homebrew/opt/node@22/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/opt/pkg/env/active/bin:/opt/pmk/env/global/bin:/opt/X11/bin:/Library/Apple/usr/bin:/Library/TeX/texbin:/usr/local/share/dotnet:~/.dotnet/tools:/usr/local/go/bin:/Library/Frameworks/Mono.framework/Versions/Current/Commands:/usr/local/bin:/opt/R/arm64/gfortran/bin:/opt/homebrew/opt/postgresql/lib:/opt/homebrew/opt/boost/lib:/opt/homebrew/opt/boost/include/boost"
  declare -x PGPASSWORD="Ybydygwnfyfamf3hduf?"
  declare -x PKG_CONFIG_PATH=":/opt/homebrew/lib/pkgconfig:/opt/homebrew/opt/openblas/lib/pkgconfig:/opt/homebrew/opt/boost/lib/pkgconfig:/opt/homebrew/opt/glibmm/lib/pkgconfig"
  declare -x PSQL_EDITOR="code -w"
  declare -x PWD="/Users/seonghobae/.config/superpowers/worktrees/design.md/css-export-20260423"
  declare -x PYENV_ROOT="/Users/seonghobae/.pyenv"
  declare -x PYENV_SHELL="bash"
  declare -x SECURITYSESSIONID="186b7"
  declare -x SHELL="/opt/homebrew/bin/bash"
  declare -x SHLVL="2"
  declare -x SSH_AUTH_SOCK="/var/run/com.apple.launchd.kou4aKbN01/Listeners"
  declare -x STRIX_LLM="openai/gpt-5.4"
  declare -x TERM="xterm-256color"
  declare -x TERM_PROGRAM="Apple_Terminal"
  declare -x TERM_PROGRAM_VERSION="470"
  declare -x TERM_SESSION_ID="9DA39B56-FB0E-470E-97B3-C0FC599C83CA"
  declare -x TMPDIR="/var/folders/vq/nxtc622x2lg1xq3jpdktymdw0000gn/T/"
  declare -x USER="seonghobae"
  declare -x VISUAL="cursor -w"
  declare -x VOOSTER_API_KEY="ak_kwjiktwryrzmf4ggtqk5fipx"
  declare -x XPC_FLAGS="0x0"
  declare -x XPC_SERVICE_NAME="0"
  declare -x _="/Users/seonghobae/.opencode/bin/opencode"
  declare -x __CFBundleIdentifier="com.apple.Terminal" command is updated to support the new format.
• Unit tests are included to verify the output.

Fixes #15
Related to #29

[google-cla]

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[seonghobae]

The implementation for the CSS custom properties export format is complete and has been fully verified locally with passing builds and tests.

The PR is currently blocked by the check, which requires manual intervention. The code itself is ready for review and merge.