feat: calibration from closed-issue actuals → forecast flips off cold-start (#1)

Close the D3 loop. The forecast now learns from the team's own estimate-vs-actual
history (the working time #5 infers from git events) instead of guessing forever.

core (@commitea/core/calibration-v0):
- fitCalibration(samples): lognormal fit on log(actual/estimate) — global +
  per-bucket (once a bucket clears the floor) + per-person bias. coldStart until
  n >= 20 closed-with-estimate issues.
- calibrationSamples(): pull those samples from the closed backlog via lifecycle
  inference (estimate label vs inferred actualWorkingDays).
- toDurationModel(): project the fit to the params forecast consumes.
- forecast() gains options.model: when past cold-start, fitted params drive the
  sim (per bucket, global fallback); otherwise the code priors do. Forecast.coldStart
  now reflects the model. nearestBucket extracted + exported.

app:
- AppShell fits calibration once from the reconciled backlog, feeds the model into
  forecastBacklog (cone), and drives the Calibration screen + Runway header.
- Focus cone footer, Runway note, and Calibration screen now say cold-start (N/20)
  vs calibrated (on N closed) from real data; Calibration scatter / bucket bias /
  per-person all fitted, degrading honestly on a thin dataset.

Known refinement: same-day closes yield 0 working-day actuals (day-granular) and
are excluded, so a fast-moving repo can sit at n=0 — honest, but a fractional
(hours-based) actual would let those count. Per-person uses gitea login, not
display name, until the person map lands.

Note: also re-lands #10 (Monte Carlo) and #5 (lifecycle) which merged into their
stacked base branches but never propagated to main (stacked-merge trap); this
branch is cut from main and carries all three so main is whole again.

Verified: 74 core tests green (9 calibration + 2 forecast-switch added), desktop
typecheck clean, 14 fixture e2e green, live spec asserts the real cold-start
calibration surface (Runway note + screen badge fitted from actuals).

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

packages/core/src/index.ts

@@ -45,5 +45,32 @@ export type {
   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'
+export {
+  COLD_START_PRIORS,
+  durationParams,
+  forecast,
+  nearestBucket,
+  PRIOR_BUCKETS,
+  priorForEstimate,
+} from './forecast/forecast-v0.js'
+export type {
+  BurnUpPoint,
+  DurationModel,
+  Forecast,
+  ForecastOptions,
+  LognormalPrior,
+} from './forecast/forecast-v0.js'
+
+export {
+  CALIBRATION_BUCKET_FLOOR,
+  calibrationSamples,
+  COLD_START_THRESHOLD,
+  fitCalibration,
+  toDurationModel,
+} from './calibration/calibration-v0.js'
+export type {
+  BucketFit,
+  CalibrationModel,
+  CalibrationSample,
+  PersonBias,
+} from './calibration/calibration-v0.js'