docs: migrate guides and snippets to v3 APIs

[zrosenbauer]

PR Checklist

• Commit message follows the repository conventions.

Bugs / Features

• Related guides, package READMEs, recipes, and rendered snippets updated.
• Existing v3 release notes aligned with the canonical APIs.
• Tests: documentation-only layer; validated with lint, formatting, generation, and source audits.

What is the current behavior?

Documentation mixes constructor-based examples, implicit startup, and older tool/workflow names with the v3 authoring surface.

What is the new behavior?

• Migrate current guides, integrations, deployment examples, recipes, package READMEs, and tutorial/landing-page code snippets.
• Document the constructor-to-factory migration, type-only exports, explicit ready(), and application shutdown.
• Align tool, workflow, and scorer documentation and release notes while retaining documented compatibility helpers.
• Update generated provider examples and their generator. Accept quoted/unquoted provider keys and single-line model tuples, preserve manually maintained provider pages, and retain the existing fallback guidance.

Testing

• Repository lint passed with six existing complexity warnings.
• Changed Markdown/MDX formatting and git diff --check passed; commit hooks passed.
• Model documentation generation completed for 80 providers and 2,195 registry models.
• Active documentation constructor audit completed; explicit migration-before examples and historical blog posts retain older syntax.
• Website build is blocked by local dependency resolution: missing webpack/lib/ContextReplacementPlugin.
• Website typecheck is blocked by missing @docusaurus/theme-classic type definitions. No dependency or lockfile changes are included.

Notes for reviewers

Layer 4 of 4, based on #3. Review and merge bottom-up: #1 → #2 → #3 → #4.

The full stack is based on next at 783491fa. The fork and local main/next were verified against upstream before publishing. All PRs are drafts in this fork.

[zrosenbauer]

Closing because this PR was mistakenly opened against the fork. Recreating it as a cross-fork draft against VoltAgent/voltagent:next; the source branch is preserved.

[zrosenbauer]

Replaced by the upstream draft: VoltAgent#1457