fix(design): add --api-timeout flag, raise default 120s to 300s for image-gen

[RagavRida]

Closes #1519.

Summary

The five image-generation callsites in the design binary all hardcoded a 120_000ms timeout with no CLI override:

• design/src/generate.ts:40
• design/src/iterate.ts:85 and :133
• design/src/variants.ts:61
• design/src/evolve.ts:55

With default settings (gpt-4o, 1536x1024, quality: high) using the Responses API with the image_generation tool, response time pushes into the 90-180s range on slower account tiers and tips over the 120s ceiling for many users. The reporter in #1519 confirmed via curl that the endpoint itself isn't slow (~26s on direct calls) and that account/network were fine — the failure is entirely the binary's hardcoded ceiling.

The existing --timeout CLI flag is plumbed only to compare --serve / serve for the HTTP listener (design/src/cli.ts:145, 251), so it does not reach the image-gen path.

Changes

• design/src/constants.ts (new): exports DEFAULT_IMAGE_GEN_TIMEOUT_MS = 300_000
• apiTimeoutMs?: number option added to GenerateOptions, VariantsOptions, IterateOptions, EvolveOptions and threaded to the AbortController callsite in each
• --api-timeout <ms> CLI flag parsed once in cli.ts and passed to all four commands. Distinct from --timeout to avoid colliding with the serve flag.
• design/test/api-timeout.test.ts (new): pins the default constant at 300_000 and verifies generateVariant honors the override via a stubbed fetch that waits on the abort signal.

Behavior

• Users who weren't currently timing out at 120s are unaffected — the path resolves the same image, just with more headroom before abort.
• Users hitting the timeout get a working default (5min headroom) plus a per-invocation override: $D generate --brief "..." --api-timeout 600000 for a 10-min ceiling.
• The variants timeout error message changed from "Timeout (120s)" to "Timeout (<n>ms)" so it stays accurate when the override is used.
• evolve's separate vision-analysis call (30s timeout, design/src/evolve.ts:113) is unchanged — that path is fast and doesn't need the headroom.

Test results

$ bun test design/test/api-timeout.test.ts
 3 pass
 0 fail
 5 expect() calls

$ bun test design/test/ test/gen-skill-docs.test.ts test/skill-validation.test.ts
 742 pass
 2 fail   # both pre-existing playwright-env failures in feedback-roundtrip.test.ts, reproduced on clean main
 6739 expect() calls

$ bun build --compile design/src/cli.ts --outfile /tmp/design-test-build
 [437ms] compile  /tmp/design-test-build

bun run gen:skill-docs was run after editing commands.ts; no SKILL.md drift (the design flag list isn't surfaced in user-visible templates).