refactor(runtime): drop unused receiver from handleStream

[dgageot]

Summary

handleStream is the chunked-stream aggregator in
pkg/runtime/streaming.go — it consumes an LLM provider's
chat.MessageStream and turns the deltas into an aggregated
streamResult plus events on the events channel.

It was declared as a method on *LocalRuntime, but its body never
touches r. The receiver is a pure artefact of where the file sits.

Drop it, tighten the events channel direction, document why.

What this is, and what it isn't

This is a pure isolation win:

• handleStream becomes a free function. It is now unit-testable
  with a mock chat.MessageStream alone — no LocalRuntime, team,
  hooks, model store, or any of the other runtime wiring required.
• events chan Event → events chan<- Event matches the rest of
  the package's send-only conventions (swapCurrentAgent etc.) and
  documents at compile time that the function never reads.
• Doc comment now spells out the dependency direction: the loop
  calls into the chunker, never the reverse.

This is not:

• A behaviour change. Same logic, same events, same return values.
• A package extraction. streamResult stays unexported in
  pkg/runtime. Lifting this into pkg/runtime/internal/streamproc
  would require inverting the events-channel dependency (Event is
  currently a runtime-defined type), which is a much larger refactor
  and explicitly out of scope here.
• A function split. handleStream is still ~200 lines doing tool-call
  aggregation, content/reasoning aggregation, usage tracking, and
  finish-reason coercion. Those concerns could be split, but that's
  a separate refactor.

Diff

Two files, six lines of substantive change:

• pkg/runtime/streaming.go: drop (r *LocalRuntime), tighten
  channel direction, add doc paragraph explaining why.
• pkg/runtime/fallback.go: drop the r. qualifier at the single
  call site.

streamResult and stripImageContent are unchanged.

Validation

• mise lint — 0 issues
• mise test — full suite passes (existing stripImageContent
  table test still works since both functions live in the same
  package)