Workflows that run the same ACP agent on several nodes had to repeat the
process attribute on every node. `acp.command` and `acp.config` were read
only from the node (`Node::acp_command_attr`), with no graph-level
fallback.
Add `AcpDefaultsTransform`, which materializes the graph-level value onto
nodes before validation. The handler's `resolve_acp_process_spec` takes
only a `Node`, so reading the graph at the use site (as `retry_target`
does) would mean threading a `Graph` through the handler. As a transform,
neither the handler nor `backend_valid` changes, and `fabro validate` and
`fabro run` stay in agreement because both go through `pipeline::transform`.
The two attributes are mutually exclusive, so they inherit as a pair: a
node setting either one keeps its own and inherits neither. Only nodes
with `backend="acp"` inherit, keeping the attributes off `start`/`exit`
and API nodes.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
