feat(core)!: introduce canonical authoring factories

[zrosenbauer]

Breaking v3 authoring APIs: defineAgent, definePlanAgent, defineTool, defineWorkflow, and voltagent. Agent state lives in closures; planning agents compose extensions. Agent, PlanAgent, and VoltAgent are type-only exports.

Validated: 1,589 core tests, 76 scorer tests, builds, source and consumer types, package checks, lint, and formatting. Changeset includes a migration example.

Layer 1 of 4, targeting next. Explicit lifecycle, examples, and documentation follow.

[changeset-bot]

🦋 Changeset detected

Latest commit: 18069f1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@voltagent/core	Major
@voltagent/scorers	Patch

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