feat: real Milestone drill-in — completes the Runway story

Clicking a milestone on Runway now opens its real detail: scope + done %, a Monte
Carlo cone over the remaining open work, and the milestone's issues grouped by
lifecycle column. Threaded the gitea milestone id through the Runway row → AppShell
→ a milestoneView().

- backlog.ts: milestoneView(id, ...) → { name, due, scope/done, forecast cone +
  range, groups by lifecycle column }. Reuses forecast + buildBurnUpData + lifecycle
  inference. null for an unknown id → the screen shows the demo fixture.
- RunwayMilestone gains an `id`; runwayView sets it; RunwayScreen.onOpenMilestone(id).
- MilestoneScreen takes optional `data`; renders real header/stats/cone/issue-groups
  when present, fixture otherwise.

Verified: desktop typecheck clean, 14 fixture e2e green. Live: clicking "P2 —
Scheduler + Monte Carlo" opens a real detail — 7 issues · est 20d, 0/7 done, cone
"80% Aug 17–26", issues in Triage/In-review from the real event stream (screenshot).

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

apps/desktop/e2e/live-backlog.spec.ts

@@ -45,6 +45,14 @@ test.describe('live backlog', () => {
     // fixture (which lists Beta / Pilot-ready / v1.0).
     await expect(win.getByText(/P2 — Scheduler/)).toBeVisible()
     await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-runway.png'), fullPage: true, animations: 'disabled' })
+
+    // Milestone drill-in — clicking a real milestone opens its real detail
+    await win.getByText(/P2 — Scheduler/).click()
+    await expect(win.getByRole('heading', { name: 'P2 — Scheduler + Monte Carlo' })).toBeVisible()
+    await expect(win.getByText(/\d+ issues · est \d+d/)).toBeVisible()
+    await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-milestone.png'), fullPage: true, animations: 'disabled' })
+    await rail.getByRole('button', { name: 'Runway' }).click()
+
     await win.getByRole('button', { name: 'Full report' }).click()
     await expect(win.getByText(/cold-start · \d+\/20|curve active · n ≥ 20/)).toBeVisible()
     await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-calibration.png'), fullPage: true, animations: 'disabled' })