fix(core): honor the host input modalities on the generate transport

[djymike]

Problem

The generate transport gated image content on the pinned capability snapshot alone:

const allowImages = modelSupportsImageInput(model.id)
if (!allowImages) assertTextOnlyMessages(context.messages)

src/commandcode-catalog.ts is generated from a single CLI release, so any model published upstream after the last sync is absent from MODEL_INPUT_MODALITIES and inputModalitiesForModel() falls back to ["text"]. The result is a transport split: the same model accepts images on the Provider API transport (which resolves image support from the host) and rejects them with Selected Command Code model does not support image content in user messages once the endpoint returns 403 upgrade_required and the request falls back to generate.

This is the mirrored counterpart of #91: the missing catalog entry degrades both capabilities, and neither has an override hook.

Change

inputModalitiesForModel(modelId, hostInput?) and modelSupportsImageInput(modelId, hostInput?) now prefer the host's resolved model.input and fall back to the snapshot when it is absent, and src/core.ts passes model.input through.

Both directions are honored: a host that advertises ["text", "image"] for a model the catalog does not list yet unlocks image content, and a host that narrows a catalogued vision model back to ["text"] keeps the rejection.

This does not replace the catalog sync (#93) — the snapshot stays the offline floor for hosts that supply no input — it removes the coupling that makes a capability override impossible without a release.

Verification

• npm run typecheck
• npx tsx tests/test-models.ts — 23 pass
• npx tsx tests/test-stream.ts — 41 pass
• npx tsx tests/test-transport.ts, npx tsx tests/test-pure-functions.ts — 5 + 49 pass

Both new tests fail on main (2 stream, 1 model) and pass with the change.

Reproduced end to end on Oh My Pi against deepseek/deepseek-v4.1-flash with a local mock endpoint: before the change the generate transport rejects the image, after it the request body carries params.messages[0].content[1].image.

Related: #90, #91, #92

[patlux]

Integrated current main; replaced the unchecked string-array cast with a type-guard filter. Unsupported host modalities cannot accidentally regain catalog image support. Added mixed/unsupported modality assertions. Confirmed 1 model test and both transport stream regressions fail on the original implementation and pass with the fix. Full npm test passes with real Pi and OMP plus format/diff checks. Waiting for CI.

[patlux]

Shipped in pi-commandcode-provider@0.7.0: https://github.com/patlux/pi-commandcode-provider/releases/tag/v0.7.0 . Public npm package model discovery and GOAT live smoke verified.