feat(budget): define model budget contract

[luoye520ww]

Problem

Kun already enforces a thread cost limit, but there is no shared contract for turn, thread, provider, agent, and workflow budgets. Different integrations could otherwise implement warning and denial semantics inconsistently.

Scope

Adds a pure model-budget contract and evaluator for the five budget scopes. It validates finite positive limits, bounded warning ratios, non-negative usage, rejects unknown fields, and returns deterministic allow/warn/deny decisions with remaining budget.

Non-goals

This PR does not change existing thread persistence, UI, model routing, provider queues, or automatic model switching. Those integrations remain separate follow-up PRs.

Tests

• npm.cmd --prefix kun test -- tests/model-budget.test.ts (8 passed)
• npm.cmd run typecheck
• npm.cmd --prefix kun run typecheck
• npm.cmd run lint
• npm.cmd run build
• git diff --check

Review

Completed functional, concurrency/lifecycle, data integrity, security, compatibility, and scope review. No package smoke is claimed because this PR is a pure contract module.

Issue

Part of #885

[luoye520ww]

Superseded by consolidated provider/network safeguards PR #948, which passed Typecheck, Linux, macOS, and Windows packaged checks. Closing this duplicate to keep the review surface focused.