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/model.go

@@ -35,10 +35,11 @@ type Stage struct {
 
 // Atlas is the full data model the frontend renders.
 type Atlas struct {
-	Version   string  `json:"version,omitempty"`
-	Substrate []Host  `json:"substrate"`
-	NS        string  `json:"ns,omitempty"`
-	Stages    []Stage `json:"stages"`
+	Version   string   `json:"version,omitempty"`
+	Substrate []Host   `json:"substrate"`
+	NS        string   `json:"ns,omitempty"`
+	Timeline  []RunDot `json:"timeline,omitempty"`
+	Stages    []Stage  `json:"stages"`
 }
 
 // Build unmarshals the authored atlas JSON and overlays generated facts from