The documented chain manifest example is the crashing duration × model combination — in USAGE.md, the module docstring, and a passing test #635

Summary

The canonical video chain manifest example we ship — in the user-facing docs, in the module docstring, and pinned by a test — is the exact combination that crashes mid-spend (#634). A user who copies the documented example loses credits and gets "Unexpected error".

The three places

1. docs/USAGE.md:847 (and its website/docs/USAGE.md mirror), under "JSONL manifest format":

{"prompt": "a lone wolf on a snowy ridge at dawn, cinematic", "model": "veo-lite", "duration": 4, "aspect": "16:9"}

veo-lite has no duration control (VideoModel.supports_duration() is True for OMNI_FLASH only), and chain.py:326 rejects omni_flash, so this line cannot succeed for any model substitution. The surrounding prose actively invites it: "model / duration / aspect are optional per-link overrides." For chains, duration is not an override — it is a crash.

2. src/gflow_cli/chain_manifest.py:5 — the module docstring carries the same line, so the in-code reference is wrong in the same way.

3. tests/test_chain_manifest.py:48 pins it as valid:

'{"prompt": "first", "model": "veo-lite", "duration": 4, "aspect": "16:9"}\n'

The test is not wrong about what it asserts — chain_manifest.py really does parse that line without complaint, because parsing is where the guard is missing. But it means the crashing combination has a green test standing behind it, which is how it survived review.

Why this is filed separately from #634

The docs fix stops the bleeding and ships in minutes; #634's guard is real work. They should not block each other. Note the two interact: once #634 lands, this example will fail at validation with exit 2 instead of mid-render — better, but still a wrong example to ship.

Suggested fix

Drop duration from the documented example and say plainly that per-link duration is unsupported in chains (rather than listing it as an optional override). Update the chain_manifest.py docstring to match, and re-point the test fixture — keeping a separate parse-level test for duration if the parser is still meant to accept the key.

Verified against develop @ e5b36ee this session.

Refs #634, #630, #632.