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/model.go

@@ -0,0 +1,68 @@
+package github
+
+import "time"
+
+// Profile is the aggregate of data other packages render into cards.
+type Profile struct {
+	ID        string
+	Login     string
+	Name      string
+	Bio       string
+	AvatarURL string
+	Company   string
+	Location  string
+	Website   string
+	CreatedAt time.Time
+
+	Followers   int
+	Following   int
+	PublicRepos int
+
+	// Totals for the stats card.
+	TotalStars         int
+	TotalForks         int
+	TotalCommits       int
+	TotalPRs           int
+	TotalIssues        int
+	TotalReviews       int
+	TotalContributedTo int
+	TotalContributions int // lifetime contributions from calendar + restricted
+
+	// Sorted desc by bytes. Color is GitHub's linguist color or "" if absent.
+	Languages []LangStat
+
+	// Commit-count histogram indexed by [day-of-week 0=Sunday][hour-of-day 0-23].
+	Productive [7][24]int
+
+	// TopRepos is the list of owned repo names sorted by stargazer count desc,
+	// populated by FetchProfile. Used as the seed set for FetchProductive.
+	TopRepos []string
+}
+
+// LangStat is one row in the top-languages card.
+type LangStat struct {
+	Name  string
+	Color string
+	Bytes int64
+}
+
+// repoNode is the GraphQL shape of one repository node; kept here because
+// it's shared by the profile fetcher and the productive-time fetcher.
+type repoNode struct {
+	Name            string `json:"name"`
+	StargazerCount  int    `json:"stargazerCount"`
+	ForkCount       int    `json:"forkCount"`
+	PrimaryLanguage *struct {
+		Name  string `json:"name"`
+		Color string `json:"color"`
+	} `json:"primaryLanguage"`
+	Languages struct {
+		Edges []struct {
+			Size int64 `json:"size"`
+			Node struct {
+				Name  string `json:"name"`
+				Color string `json:"color"`
+			} `json:"node"`
+		} `json:"edges"`
+	} `json:"languages"`
+}