docs: fresh README hero screenshot showcasing the v2.6 dashboard #19

Problem

The README screenshot shows the v2.5 dashboard. After PR #17 (v2.6.0 "Dashboard Elevated" — boot sequence, ⌘K palette, tmux status bar, mission track, task burn), the headline image undersells the product's best work.

Plan (agreed with Daniel)

When Daniel picks this up, Claude will:

1. Build a showcase dashboard-data.js — not the test fixture, but a deliberately beautiful, realistic project: interesting mission, 4–5 phases with rich specs/designs/tasks, a meaty architecture diagram, mixed phase states so the mission track shows complete + active-pulsing + roadmap nodes, partial DoD for the LED bar, and stack rows exercising all four source badges
2. Recommend the exact shot(s) to capture — which view, which state, window size — optimizing for the most beautiful screenshot (current candidates: Overview with mission track + task burn + tmux bar all visible; the boot sequence mid-type as a secondary/GIF shot; a phase design tab with the architecture map)
3. Daniel takes the screenshot and swaps it into the README

Acceptance

• Showcase data file created
• Shot list with framing guidance delivered
• README image replaced (and alt text updated to describe the v2.6 shell)

Origin: v2.6.0 release-process audit (PR #17).