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.

internal/github/queries.go

@@ -0,0 +1,77 @@
+package github
+
+// profileQuery pulls everything needed for the profile, stats and languages
+// cards in one round trip. Repo pagination is handled by the caller if the
+// user owns more than 100 repos.
+const profileQuery = `
+query($login: String!, $after: String) {
+  user(login: $login) {
+    id
+    login
+    name
+    bio
+    avatarUrl
+    company
+    location
+    websiteUrl
+    createdAt
+    followers { totalCount }
+    following { totalCount }
+    pullRequests { totalCount }
+    issues { totalCount }
+    repositoriesContributedTo(
+      first: 1
+      contributionTypes: [COMMIT, PULL_REQUEST, ISSUE, PULL_REQUEST_REVIEW]
+    ) { totalCount }
+    contributionsCollection {
+      totalCommitContributions
+      totalIssueContributions
+      totalPullRequestContributions
+      totalPullRequestReviewContributions
+      totalRepositoryContributions
+      restrictedContributionsCount
+      contributionCalendar { totalContributions }
+    }
+    repositories(
+      first: 100
+      after: $after
+      ownerAffiliations: OWNER
+      isFork: false
+      orderBy: { field: STARGAZERS, direction: DESC }
+    ) {
+      totalCount
+      pageInfo { hasNextPage endCursor }
+      nodes {
+        name
+        stargazerCount
+        forkCount
+        primaryLanguage { name color }
+        languages(first: 20, orderBy: { field: SIZE, direction: DESC }) {
+          edges {
+            size
+            node { name color }
+          }
+        }
+      }
+    }
+  }
+}`
+
+// commitHistoryQuery fetches commit timestamps in the default branch of one
+// repo, filtered to commits authored by the target user. Used to build the
+// productive-time heatmap.
+const commitHistoryQuery = `
+query($login: String!, $repo: String!, $userId: ID!, $since: GitTimestamp!, $after: String) {
+  repository(owner: $login, name: $repo) {
+    defaultBranchRef {
+      target {
+        ... on Commit {
+          history(first: 100, after: $after, author: { id: $userId }, since: $since) {
+            pageInfo { hasNextPage endCursor }
+            nodes { committedDate }
+          }
+        }
+      }
+    }
+  }
+}`