feat: Monte Carlo forecast → real burn-up cone (#10)

Replace the demo cone on Morning service with a real, seeded Monte Carlo
forecast over the open backlog. The LLM never does this — it's plain,
reproducible code (evidence-based scheduling).

core (@commitea/core/forecast-v0):
- Code-resident lognormal cold-start priors per estimate bucket (D3):
  sampled actual = estimate * exp(N(mu, sigma)), mu > 0 (actuals run long),
  sigma shrinks as tickets grow. Replaced by the team's empirical fit at
  n >= 20 (#5 supplies the actuals).
- forecast(): seeded mulberry32 + Box-Muller over the scheduler's
  deterministic order (order is fixed from estimates/deps; only durations
  vary, so the cone stretches, never reorders). Returns p50/p80/p95 landing
  + a per-issue burn-up curve (p10/p50/p90). 12 unit tests; reproducible.

renderer:
- lib/dates.ts: working-day -> calendar mapper (skips weekends) + buildBurnUpData.
- BurnUpCone gains a data-driven twin; falls back byte-identical to the
  fixture cone when no forecast (demo mode unchanged).
- Focus card shows the real "80% of the open backlog lands by <range>",
  real scope count, and names the cold-start priors.

v0 scope (each a later slice): single serial worker (capacity is #8);
cold-start priors only (empirical fit is #5); no historical actual polyline
(needs lifecycle events, #5). Header chrome (reconcile time, ahead/behind
badge) stays fixture until milestone due dates land.

Verified: 51 core tests green, desktop typecheck clean, 14 fixture e2e green,
live spec asserts the real cone renders (25 open issues, "lands by Nov 11-27").

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

packages/core/src/index.ts

@@ -33,3 +33,6 @@ export type {
   ScheduledItem,
   SchedulePlan,
 } from './scheduler/scheduler-v0.js'
+
+export { COLD_START_PRIORS, forecast, priorForEstimate } from './forecast/forecast-v0.js'
+export type { BurnUpPoint, Forecast, ForecastOptions, LognormalPrior } from './forecast/forecast-v0.js'