feat(ai)!: upgrade to AI SDK v7 (parity + seam hardening)

[bratsos]

Summary

Upgrades the AI layer from AI SDK v6 → v7 (ai@7, @ai-sdk/provider@4, @ai-sdk/google@4, @openrouter/ai-sdk-provider@3). This is the parity + seam-hardening pass — zero intended runtime behavior change; the follow-up PR adds the new consumer-facing surface (usage detail, toolApproval, telemetry, etc.).

Targets 0.12 and is independent of the pending 0.11 release.

Approach

A measurement spike first established that the official @ai-sdk/codemod does net harm here — it renames object properties by name and half-renamed our own internal types. So the codemod output was discarded and the real changes hand-applied. The measured break surface turned out small: v7 keeps system/onFinish/onStepFinish/totalUsage as working deprecated aliases, so only a handful of things actually needed to change.

Breaking changes (TypeScript; pre-1.0 minor)

• ProviderResolver and registerEmbeddingProvider now type against LanguageModelV4/EmbeddingModelV4 (the @ai-sdk/provider@4 interface). Consumers passing their own provider must supply a v7 provider.
• TextOptions.experimental_output → output (the one v7 param renamed with no alias).
• onStepFinish → onStepEnd on TextOptions/ObjectOptions/StreamOptions.
• streamText input system → instructions.

The batch API's own system field is unchanged (it's not the streamText system). No runtime behavior change; cost/usage accounting is identical.

Seam hardening

• The stream onFinish callback (previously an untyped hand-written inline shape reading totalUsage) is now onEnd typed with the SDK's exported GenerateTextEndEvent<ToolSet>, reading event.usage with a totalUsage fallback — so a future SDK shape change surfaces at compile time.
• Removed the two aiGenerateText(... as any) casts; they now use the typed as Parameters<typeof aiGenerateText>[0] (still needed to dodge NoInfer<TTools>, but no longer any).
• getReasoning() prefers result.finalStep.reasoningText (v7 canonical) with the deprecated top-level as fallback.

Extra fix caught during migration

v7 widened Tool.description to string | ((options) => string) (dynamic descriptions). Batch tool defs are static, so anthropic-batch.ts now coerces tool.description to a string.

Verification

• pnpm -r typecheck clean across all four packages.
• Tests: core 1095 passed / 1 skipped (matches main), host packages 118 passed.
• Lint clean.

Docs

• Changeset (ai-sdk-v7.md, minor).
• New migration guide migrate-0.11-to-0.12.md (mirrors the 0.10→0.11 structure) covering the four renames and the provider-interface change for custom ProviderResolver/embedding providers.
• Skill reference 04-ai-integration.md updated to v7 names.

Not in this PR

The consumer-surface expansion (full usage detail → cost ledger, toolApproval → Approval conventions, telemetry → OTel, result richness, messages on text/object, unified reasoning option) lands as a separate PR stacked on this one.