akb v0.19.1

Fixed

• akb init stages and commits the new base through the index-lock retry. A concurrent
  process holding the git index lock no longer aborts the initial staging of base files;
  the retry now covers both stages.
• The compute-budget error is identified by sentinel, not by text. internal/cel
  exports ErrComputeBudget, and a rule's evaluation error embeds the key names the rule
  reads — so text matching handed the budget remedy to rules whose field names collide with
  the phrase. Write-path dispatch now matches with errors.Is.
• akb template get --example warns about every broken mockup rule, not just the first.
  Revalidation previously stopped at the first unevaluable rule, hiding later failures and
  dropping the variant's staleness headline. Every failed rule is collected, then named.
• The template-load failure is the error reported, not a stale page-read error. When
  akb index add falls back to templates and the template set fails to load, the wrapped
  load error is surfaced — it names the cause where the old message named only the page;
  the per-file guard at the append, index-add, lint, and type-directory call sites keeps
  its own error, its classification, and its exit code.