Replaces the Multi FileFeed's HEAD flowgraph with the supplied state.

The flowgraph.nodes and flowgraph.edges arrays represent the complete
desired state — nodes absent from the payload are deleted, and edges are
fully replaced. Every call must include a source node and an import or
import-v2 node or the request will be rejected. Represent the complete
workflow as a single graph.

Within an individual node, config uses merge semantics:

If a key is not specified, the current value is left unchanged.
If a key takes a scalar value

• a supplied value replaces the current value
• a null value deletes the key
  If a key takes an array value
• supplied value replaces the current array
• a null value deletes the key
  If a key takes a hash/object value
• the supplied hash is recursively merged with the current hash (same rules apply at each nested level)
• a null value deletes the key

Auto-commits by default. Use commit_now: false to defer committing.

Concurrency guards:

• last_known_commit_id — optimistic lock; 409 if commits changed
• Dirty-state check — 409 if HEAD has uncommitted UI edits (override with force: true)

Requires Multi FileFeed transforms to be enabled for the organization.