feat(atlas): UX — overview rail + gate glyphs + pill reconcile (#3,#4,#5)

Closes the UX sprint's authored layer.
#3 Horizontal-scroll blindness → an always-visible "PIPELINE" overview rail:
9 colour-coded chips (00 Notice → 08 Learn) showing the whole shape at a glance,
each click-to-scroll to its stage. First-timers now see the arc + that more exists.
#4 Real gate glyph — 🔒 on the two checkpoint transitions (human @04, CI @06),
desktop spine + mobile rows.
#5 Reconcile legend with Plain: decorative node pills hidden in Plain, so colour
is reserved for live status (green/amber/coral) — exactly what the legend keys.
(#6 attached chips/grammar: the question-vs-statement label mix is meaningful —
decisions vs milestones — kept; SVG chip background skipped as low-value.)

New Stage.short (guarded by JSON validity via existing tests). build/vet/lint(0)/
test green; desktop Plain screenshot-verified.

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

internal/atlas/model.go

@@ -32,6 +32,7 @@ type Node struct {
 // Nodes are derived from a live source at Build time.
 type Stage struct {
 	No         string `json:"no"`
+	Short      string `json:"short,omitempty"`
 	Title      string `json:"title"`
 	PlainTitle string `json:"plain_title,omitempty"`
 	Plain      string `json:"plain,omitempty"`