In-session phase presentation: header chip + dotted boundary dividers

- components/phase-divider.tsx: PhaseChip (header, next to the session
  title: phase-colored dot + Probing/Planning/Teaching) and PhaseDivider
  (the transcript boundary: color-coded dotted rules + a small chip —
  'Plan · N nodes' at the plan seam, 'Teaching' at the teach seam; probe
  never renders, phases only advance). planNodeCount reads the seam
  message's first plan tree.
- chat.tsx: dividers render at MESSAGE boundaries (phaseTimeline, one
  linear fold) — the seam is the transition to the message's phase, so
  the seam message's prose sits below its divider. requiresReveal now
  uses phaseAt3 teach (an occupied node), so probe- AND plan-phase
  questions auto-open.
- Retired the part-level plan-divider (artifact registry): the boundary
  is message-level, not part-level. Its tests replaced by
  render-phase-divider tests; core/phases gains tested phaseTimeline.

components/artifacts/index.ts

@@ -2,7 +2,6 @@ import type { ComponentType } from "react";
 import type { ArtifactPartProps } from "@/src/core";
 import { DemoCard } from "@/components/artifacts/demo-card";
 import { JobCard } from "@/components/artifacts/job-card";
-import { PlanDivider } from "@/components/artifacts/plan-divider";
 import { VisualCard } from "@/components/artifacts/visual-card";
 
 /**
@@ -19,8 +18,9 @@ import { VisualCard } from "@/components/artifacts/visual-card";
  */
 export const artifactRenderers: Record<string, ComponentType<ArtifactPartProps>> = {
 	research: JobCard,
-	// Phase divider where the plan lands (W5 fills in the component):
-	plan_init: PlanDivider,
+	// plan_* parts render nothing inline by design: the phase seam is a
+	// message-level boundary, drawn by PhaseDivider in chat.tsx (the old
+	// part-level plan_init divider retired with it).
 	// Artifacts (W6 fills in the components):
 	demo: DemoCard,
 	visual: VisualCard,