tasks(decomp): ADR-049 streaming handler — 8 atomic tasks + gitignore .worktrees/

Decompose the ADR-049 streaming handler work into 8 dependency-ordered tasks:
- call/registry/streaming-handler-handlerkind (foundation: StreamingHandler,
  HandlerKind, ResponseStream, INVALID_OPERATION_TYPE, migrate all sites)
- call/registry/invoke-streaming (OperationRegistry::invoke_streaming)
- call/protocol/dispatch-streaming-branch (server-side op_type branch)
- call/client/from-call-streaming-forwarding (Subscription → subscribe())
- http/gateway/invoke-streaming (GatewayDispatch::invoke_streaming)
- http/server/subscribe-sse-streaming (/subscribe pipes BoxStream to SSE)
- http/adapters/from-openapi-sse-streaming (SSE → StreamingHandler)
- review-streaming-impl (phase review checkpoint)

Validated with taskgraph: 86 tasks, no cycles. Also ignore .worktrees/ so
agents' worktree workspaces don't leak into git status.

.gitignore

@@ -1,2 +1,3 @@
 target/
-node_modules/
+node_modules/
+.worktrees/