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/productive.go

@@ -0,0 +1,81 @@
+package github
+
+import (
+	"time"
+)
+
+// productiveGQL is the response shape for commitHistoryQuery.
+type productiveGQL struct {
+	Repository *struct {
+		DefaultBranchRef *struct {
+			Target *struct {
+				History struct {
+					PageInfo struct {
+						HasNextPage bool   `json:"hasNextPage"`
+						EndCursor   string `json:"endCursor"`
+					} `json:"pageInfo"`
+					Nodes []struct {
+						CommittedDate string `json:"committedDate"`
+					} `json:"nodes"`
+				} `json:"history"`
+			} `json:"target"`
+		} `json:"defaultBranchRef"`
+	} `json:"repository"`
+}
+
+// FetchProductive fills p.Productive with a [7][24] commit histogram over the
+// last year, gathered from the user's top-starred owned repos. Each repo is
+// sampled up to maxPerRepo commits to keep the cost bounded.
+//
+// The timezone loc is applied to CommittedDate so the heatmap reflects when the
+// user actually commits, not UTC.
+func (c *Client) FetchProductive(p *Profile, repos []string, loc *time.Location, maxPerRepo int) error {
+	if loc == nil {
+		loc = time.UTC
+	}
+	since := time.Now().AddDate(-1, 0, 0).UTC().Format(time.RFC3339)
+
+	for _, repo := range repos {
+		var cursor *string
+		seen := 0
+		for {
+			if seen >= maxPerRepo {
+				break
+			}
+			vars := map[string]any{
+				"login":  p.Login,
+				"repo":   repo,
+				"userId": p.ID,
+				"since":  since,
+			}
+			if cursor != nil {
+				vars["after"] = *cursor
+			}
+
+			var resp productiveGQL
+			if err := c.query(commitHistoryQuery, vars, &resp); err != nil {
+				return err
+			}
+			if resp.Repository == nil || resp.Repository.DefaultBranchRef == nil ||
+				resp.Repository.DefaultBranchRef.Target == nil {
+				break
+			}
+			h := resp.Repository.DefaultBranchRef.Target.History
+			for _, n := range h.Nodes {
+				t, err := time.Parse(time.RFC3339, n.CommittedDate)
+				if err != nil {
+					continue
+				}
+				tl := t.In(loc)
+				p.Productive[int(tl.Weekday())][tl.Hour()]++
+				seen++
+			}
+			if !h.PageInfo.HasNextPage {
+				break
+			}
+			end := h.PageInfo.EndCursor
+			cursor = &end
+		}
+	}
+	return nil
+}