vibe scene build: multi-provider T2I routing (Gemini + Grok) + fix help text

[kiyeonjeon21]

Why

ROADMAP Phase 4 lists "Multi-provider T2I in scene build" — currently the help text claims OpenAI-only but the code already supports Gemini, and Grok routing is missing entirely.

State of code (2026-04-29)

• packages/cli/src/commands/scene.ts:1256 — vibe scene build --image-provider <name> help reads "openai (only one supported in v0.60)". This is misleading — the underlying executor at line 894 reads (opts.imageProvider ?? "gemini").toLowerCase() and the validator at line 896 already accepts both gemini and openai.
• packages/cli/src/commands/scene.ts:519 — vibe scene add --image-provider <name> correctly advertises "Image provider: gemini, openai".
• No Grok / xAI image provider routing in either subcommand.

So today: scene build --image-provider gemini actually works, the user just isn't told. And Grok is unreachable.

Scope

• Update scene build help text (scene.ts:1256) to match actual support: "Image provider: gemini, openai, grok" (or whatever ships)
• Add Grok image provider branch in the asset-generation block at scene.ts:889-920 (mirror the OpenAI branch — XAI_API_KEY env, GrokImageProvider from @vibeframe/ai-providers)
• Make --image-provider an enum (commander .choices() or zod) so invalid values fail at parse time instead of asset-gen time
• Tests: _shared/scene-render.test.ts style — dry-run snapshot for each provider
• --describe envelope (CLI UX audit: --json schema, --describe coverage, exit codes #33) should expose the new enum values

Out of scope

• I2V (motion-video backdrops) — tracked separately

Reference

• ROADMAP.md Phase 4 "Open items in Phase 4 (v0.61+ candidates)"
• Asset-gen block: packages/cli/src/commands/scene.ts:889-920

[kiyeonjeon21]

Maintainer note: the goal is still valid, but the issue should be read against the current vibe build path rather than the older line references.

Current state:

• lower-level vibe scene add --image-provider supports gemini and openai
• top-level vibe build --image-provider still documents openai
• _shared/scene-build.ts still rejects non-OpenAI backdrop generation for build assets
• build planning warns when the resolved image provider is not supported by build assets

So the remaining work is specifically multi-provider backdrop generation for vibe build assets, plus keeping the CLI help / manifest / plan validation in sync. Contributors should not spend time only updating the old scene.ts help text from the original issue body.

A current implementation should touch the build path roughly around:

• packages/cli/src/commands/build.ts
• packages/cli/src/commands/_shared/scene-build.ts
• packages/cli/src/commands/_shared/build-plan.ts
• packages/cli/src/tools/manifest/scene.ts
• related dry-run / envelope / scene-build tests