engine.env can't be shared: rejected in engine definitions and imports; behaviors.execution.env silently drops the api-proxy target #60858

Context: orgs running many workflows on one custom engine (a shared definition file imported via imports:) need engine-wide env (endpoint URL + install knobs) defined once. Today every consumer must copy-paste the same engine.env block.

What I tried (gh-aw v0.89.10):

1. env: under engine: in the definition file → schema rejects the whole file: Unknown properties: behaviors, description, display-name, experimental, provider (the moment env appears, the definition's own documented keys become "unknown").
2. A separate import fragment whose frontmatter carries only engine.env → rejected: Unknown property: env. 'env' belongs under 'container', 'safe-outputs' or 'sandbox/agent'.
3. behaviors.manifest.execution.env in the definition → compiles green but silently loses the api-proxy target: the compiled lock's apiProxy.targets.openai.host comes out EMPTY (the engine env points at the local gateway with no upstream). Target extraction apparently reads only the consumer's engine.env literal.

Asks:

• Allow engine.env in definition files and/or import fragments (deep-merged with the consumer's), including compile-time api-proxy target extraction from the merged value.
• Independently: when no target is extractable for an enabled proxy adapter, that should be a compile error, not a silent empty target — failure mode 3 above produced a green compile and a broken proxy.

Workaround for now: keep the literal in every consumer's engine.env (documented per-consumer contract).