Add a `max-tokens` variant to `TurnEndReasonMap` and carry the model
finish reason up from `runStep` to `runTurn`, applying the rule "any
max-tokens step in the turn surfaces as max-tokens" (disposed/aborted/
error still take precedence). This lets consumers distinguish a clean
stop from a truncated one — the contract RFC 010's ACP bridge maps to
the `max_tokens` stop reason.
Also add an AGENTS.md rule: write an ADR when (and only when) a PR makes
a durable, contested, surprising decision.
Tianyi Cui
add59a3336