feat(builder): run codegen + preview on any configured LLM provider (#297)

[MarvinVomberg]

Closes #297.

Summary

Makes the Agent Builder provider-agnostic. The Builder was the last place still hard-locked to Anthropic: modelRegistry.ts rejected any non-Anthropic registry entry, and both the builder agent loop and the preview chat constructed an Anthropic provider at the call site. Building on the pluggable provider registry (#298) and per-instance orchestrator models (#296), this lets a draft run codegen and preview on any model whose provider the operator has configured.

Changes

• modelRegistry: removes the Anthropic-only guard; sources models from the global @omadia/llm-provider registry. New resolve(ref) → { provider, modelId } throws a clear, actionable error for an unregistered reference. Anthropic slugs stay as the default and as a pre-boot fallback.
• types / draftStore: BuilderModelId is now any registry reference (provider-qualified id or legacy slug). The stored value is preserved as-is rather than coerced to a default, so a draft pinned to a removed provider fails loudly at run time instead of being silently reset.
• builderAgent + previewChatService: the anthropic: () => AnthropicClient dependency is replaced by a resolveProvider seam. The LocalSubAgent is built from the resolved LlmProvider + bare vendor id. Resolution happens per turn, so a hot-swapped key (Setup Wizard / admin secrets) is picked up without a restart.
• index.ts: resolveBuilderProvider wires the Anthropic fast-path (shared hot-swappable client) plus the generic resolveLlmProvider path (vault scope + provider catalog). builderConnectedProviders feeds the picker so only providers with a configured API key (or keyless providers like Ollama) are selectable.
• routes: GET /models lists models from every connected provider; builderChat / builderPreview validate against the registry and pass the reference through; builderPreviewPrompt resolves the reference for persona-family inference.
• web-ui: the Codegen/Preview model picker is populated from /models, grouped per provider (<optgroup>), instead of the fixed haiku | sonnet | opus set.

Acceptance criteria (#297)

• A draft can run codegen and preview on a non-Anthropic model when that provider is configured.
• The Anthropic-only guard in modelRegistry.ts is gone; no code path assumes the Builder model is Anthropic.
• The picker lists models from every configured provider, not a fixed three-slug set (and hides providers without a key).
• New drafts default to an Anthropic model; existing drafts keep their stored choice.
• A draft referencing an unregistered model fails with a clear, actionable error.

Testing

• npm run typecheck (middleware + web-ui) clean.
• Full middleware suite: 3425 pass / 0 fail. Builder tests migrated to the new resolveProvider seam.
• ESLint clean on all changed files.
• Manual smoke-check in a local container stack: a draft whose stored codegen_model was mistral:mistral-large-latest completed its builder-chat turns, while drafts left on the Anthropic default returned a 401 from an invalid local Anthropic key. Reviewers should run their own end-to-end check with valid provider keys.

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