fix(imagegen): bump image-to-image timeout to 180s (was 60s shared with t2i)

[KillerQueen-Z]

Summary

The 60s setTimeout on the main /v1/images/{generations,image2image} fetch in src/tools/imagegen.ts covers both x402 retry attempts AND the actual generation. That budget is generous for text-to-image but consistently insufficient for image-to-image with gpt-image-2.

Root cause

```typescript
const controller = new AbortController();
const timeout = setTimeout(() => controller.abort(), 60_000); // 60s timeout
```

For text-to-image (gpt-image-1, ~10–30s) — fine. For image-to-image (gpt-image-2):

• gpt-image-2 is reasoning-driven; an edit easily takes 60–150s on its own.
• The request body carries a few MB of base64 reference image, adding measurable upload time.
• The timeout shares its budget across the 402 round trip plus the paid retry.

End result: every gpt-image-2 image-to-image call hits `AbortError` before upstream returns, with a misleading error message ("Image generation timed out (60s limit). Try a simpler prompt.") that made it look like a server problem.

Reproduction

```bash
franklin
/model sonnet

use openai/gpt-image-2 with reference image /any/normal/photo.png to generate
```

Hangs at AbortError every time on a 1–4MB reference image.

Fix

Split the budget by mode:

• image-to-image: 180s (3 minutes — comfortably covers worst-case gpt-image-2 edits)
• text-to-image: 60s (unchanged)

Also updates the timeout error message so users know the longer limit applies and gives more relevant remediation hints (smaller image / simpler prompt) when the reference-image path was taken.

Out of scope

• The existing 30s timeouts on `resolveReferenceImage` (URL fetch) and on the result download stay unchanged — those are network-bound and shouldn't routinely be slow.
• No change to text-to-image behavior, x402 flow, or the request schema.

Test plan

1. Pick any 1–4MB PNG.
2. `franklin` → `/model sonnet`
3. `use openai/gpt-image-2 with reference image /path/to/img.png to generate something`
4. Before this patch: AbortError after ~60s every time.
5. After this patch: completes in 60–120s typically; only hits the new 180s ceiling under truly degraded conditions.

```
src/tools/imagegen.ts | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
```