feat(base)!: one budget per person for all their assistants, the effort each takes, the operator's lists by page

A grant now says only which assistant a person may use, and keeps their
agreement; the limits are the person's (assistant_budget): one per UTC
day and month in money and in tokens, each optional, money counting the
assistants with prices, tokens every one (Halkyon Learn's ADR 0021, D58,
D59). assistantOperator.grant gives several people several assistants
with one budget, change sets one person's exactly, revoke takes back
everything; grantees and granteesOf list people with their assistants,
budget and use, 50 at a time. The migration carries each person's grant
limits into a budget: per unit, their sum, a grant without a limit making
it none.

assistants.env gives each assistant a default and a highest effort
(_EFFORT, _MAX_EFFORT; refused at start where the provider takes none);
ask takes one within that range, refusing another (invalidEffort), and
caps the output at its ceiling (D63).

instanceOperator.waiting and accounts come 50 at a time from an offset,
and counts gives the rows of the operator's screen (D62).

BREAKING CHANGE: the offer, the grants and the operator's lists changed
shape; Learn's server now asks for build 25.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_server/lib/src/instance/app_compatibility.dart

@@ -26,4 +26,9 @@
 /// 24 since GitHub signs nobody in (ADR 0020, D36, decided 2026-10-25):
 /// builds up to 23 (v0.20.0) read whether an account has GitHub
 /// (`SignInWays.gitHub`), which is gone; 24 doesn't.
-const minimumAppBuild = 24;
+///
+/// 25 since a budget is a person's, not each grant's (ADR 0021, D58,
+/// decided 2026-10-26): builds up to 24 (v0.21.0) read each assistant's
+/// limits from `assistant.offered` and grant with `assistantOperator.grant`
+/// per assistant, both changed; 25 reads the person's budget.
+const minimumAppBuild = 25;