Streamed tool-call JSON can deliver `_i` as an object, number, or boolean
before schema validation lands. The optional-chain in
#updateWorkingMessageFromIntent only guarded null/undefined, so any
non-string value crashed the UI with 'intent?.trim is not a function'.
Accept `unknown` and bail unless typeof intent === 'string', mirroring
the typed guard in agent-loop's extractIntent. Drop the misleading
`as string | undefined` cast at the streaming call site.
Fixes#900
can1357
cb0d51b0f1