feat(image): add --seed for reproducible generations

[mvanhorn]

Summary

Adds --seed <n> to ai image. The flag is forwarded to generateImage as the SDK's top-level seed option, so providers that honor it (BFL, Stability, fal, and other supported models) return reproducible images for a given (prompt, model, seed) tuple.

Why this matters

The repo's tagline is "predictable artifact outputs" but ai image has no way to ask for a specific seed today. Seeded generation is the canonical reproducibility primitive across most image CLIs (replicate, fal, the Stable Diffusion UIs). With this flag, prompt iteration becomes a clean diff on the prompt rather than two changing variables.

Demo

Simulated demo. Real image generation needs an AI Gateway key, so the demo uses placeholder gradients to make the contrast visible:

Without --seed, two runs of the same prompt produce different outputs. With --seed 42, two runs produce identical outputs.

Changes

• packages/ai-cli/src/commands/image.ts: add --seed <n> option, parse via parsePositiveInt, forward as the SDK's top-level seed argument. Print a warning on stderr when every selected model is OpenAI (gpt-image ignores seed). Print a separate warning when --seed is combined with -n > 1, since the generations would be identical on supported providers.
• packages/ai-cli/src/commands/completions.ts: add --seed to IMAGE_FLAGS and the zsh image flag list so tab completion picks it up in bash, zsh, and fish.
• packages/ai-cli/README.md: add the --seed <n> row to the image flag block and a short note describing the SDK pass-through, the OpenAI exception, and the -n > 1 caveat.

Testing

bun run typecheck                            # passes
bun run --cwd packages/ai-cli format:check   # passes
bun run --cwd packages/ai-cli lint           # 24 pre-existing warnings in lib/openh264.mjs, 0 errors
bun test                                     # 71 tests, 0 fail (1 new in cli.test.ts)

The new test asserts ai image --seed abc x exits 1 with --seed must be a positive integer. The two warning paths follow the existing --quality/--style warning shape, so no new coverage was added for them.

buildProviderOptions stays the only place provider-specific keys are assembled; the OpenAI quality/style branch is unchanged.

[vercel]

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[vercel]

--seed 0 is incorrectly rejected because parsePositiveInt requires n > 0, but 0 is a valid seed value for image generation APIs.