feat(atlas): Phase C — recent-runs timeline

A strip of the last 12 runs (aggregate pass/fail/running per run) below the
substrate ribbon, coloured, newest-first, run # on hover. Parsed from the same
Gitea /actions/tasks fetch (RecentRuns, test-first; State refactored to share
the aggregate). No new RBAC. Hidden when no live data.

Verified: build/vet/lint(0)/test green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/atlas/runs.go

@@ -31,8 +31,50 @@ type RunSummary struct {
 // State aggregates the jobs: failure if any failed, running if any not yet
 // succeeded, else success.
 func (s RunSummary) State() string {
+	return aggregateState(s.Jobs)
+}
+
+// RunDot is one run's aggregate outcome for the recent-runs timeline.
+type RunDot struct {
+	Number int    `json:"number"`
+	State  string `json:"state"`
+}
+
+// RecentRuns parses a Gitea `/actions/tasks` response (per-job, newest first)
+// into up to n most-recent runs with their aggregate outcome, newest first.
+func RecentRuns(tasksJSON []byte, n int) ([]RunDot, error) {
+	var resp struct {
+		Tasks []struct {
+			RunNumber  int    `json:"run_number"`
+			Status     string `json:"status"`
+			Conclusion string `json:"conclusion"`
+		} `json:"workflow_runs"`
+	}
+	if err := json.Unmarshal(tasksJSON, &resp); err != nil {
+		return nil, fmt.Errorf("parse tasks: %w", err)
+	}
+	var order []int
+	jobsByRun := map[int][]Job{}
+	for _, t := range resp.Tasks {
+		if _, seen := jobsByRun[t.RunNumber]; !seen {
+			order = append(order, t.RunNumber)
+		}
+		jobsByRun[t.RunNumber] = append(jobsByRun[t.RunNumber], Job{Status: t.Status, Conclusion: t.Conclusion})
+	}
+	dots := make([]RunDot, 0, n)
+	for _, rn := range order {
+		if len(dots) >= n {
+			break
+		}
+		dots = append(dots, RunDot{Number: rn, State: aggregateState(jobsByRun[rn])})
+	}
+	return dots, nil
+}
+
+// aggregateState folds per-job outcomes into a run outcome.
+func aggregateState(jobs []Job) string {
 	allSucceeded := true
-	for _, j := range s.Jobs {
+	for _, j := range jobs {
 		switch j.State() {
 		case "failure", "cancelled", "error":
 			return "failure"