feat: implement profile summary cards with GraphQL fetch and Action wrapper

- Add GraphQL client fetching profile, stats, language aggregation, and
  per-repo commit histograms for the productive-time heatmap.
- Render real SVG cards (profile details, top languages, stats grid,
  weekday×hour heatmap) with XML escaping and thousands-formatted numbers.
- Expand theme palette to 30 built-ins ported from github-readme-stats;
  add -list-themes, multi-theme rendering, and 'all' shortcut.
- Package as Docker-based GitHub Action (action.yml, Dockerfile,
  entrypoint.sh) with optional auto-commit of generated cards.
- Release workflow publishes GHCR image and cross-platform binaries on
  v* tags.
- Unit tests cover rendering, XML escape, number formatting, language sort.

entrypoint.sh

@@ -0,0 +1,58 @@
+#!/bin/sh
+# Entrypoint for the ghstats GitHub Action.
+# Inputs are passed via INPUT_* environment variables (set by the Action runtime).
+# This script translates them into ghstats CLI flags and optionally commits the
+# generated SVGs back to the repository.
+
+set -eu
+
+user="${INPUT_USER:-}"
+token="${INPUT_TOKEN:-${GITHUB_TOKEN:-}}"
+out="${INPUT_OUT:-output}"
+themes="${INPUT_THEMES:-dracula}"
+tz="${INPUT_TZ:-UTC}"
+top_repos="${INPUT_TOP_REPOS:-10}"
+commits_per_repo="${INPUT_COMMITS_PER_REPO:-100}"
+commit_changes="${INPUT_COMMIT_CHANGES:-false}"
+commit_message="${INPUT_COMMIT_MESSAGE:-chore: update ghstats cards}"
+commit_branch="${INPUT_COMMIT_BRANCH:-}"
+author_name="${INPUT_AUTHOR_NAME:-github-actions[bot]}"
+author_email="${INPUT_AUTHOR_EMAIL:-41898282+github-actions[bot]@users.noreply.github.com}"
+
+if [ -z "$user" ]; then
+  echo "::error::input 'user' is required" >&2
+  exit 2
+fi
+
+mkdir -p "$out"
+
+echo "Running ghstats for user=$user themes=$themes out=$out"
+ghstats \
+  -user "$user" \
+  -token "$token" \
+  -out "$out" \
+  -themes "$themes" \
+  -tz "$tz" \
+  -top-repos "$top_repos" \
+  -commits-per-repo "$commits_per_repo"
+
+if [ "$commit_changes" = "true" ]; then
+  workspace="${GITHUB_WORKSPACE:-/github/workspace}"
+  cd "$workspace"
+  git config --global --add safe.directory "$workspace"
+  git config user.name "$author_name"
+  git config user.email "$author_email"
+
+  if [ -n "$commit_branch" ]; then
+    git fetch origin "$commit_branch" || true
+    git checkout -B "$commit_branch"
+  fi
+
+  git add "$out"
+  if git diff --cached --quiet; then
+    echo "No card changes to commit."
+  else
+    git commit -m "$commit_message"
+    git push origin HEAD
+  fi
+fi