feat(cli): surface learn spend, the savings ledger, and holdout reports

Add the CLI half of learn v3: a window-cost and effective-input line on the
default report, `learn savings`, `learn experiment start|arm|report|list|stop`,
`learn export`, and `learn reconcile --usage-export <csv>`.

The rendering enforces the same rules the analyzer does structurally. Savings
rows are grouped by measurement method with no blended headline, confounders and
provenance print in the default view rather than behind a flag, and a confirmed
row without attribution is shown as unattributed rather than attributed by
default. Money is formatted down to five decimals so real sub-cent spend does not
round to $0.00 and read as nothing.

`export` and `reconcile` stay machine-readable: the digest is a file to read
before deciding to share it, and a reconciliation is a table, not a headline.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>