Refresh planner queue after pgx migration

[asim]

Summary:

• Drop the closed pgx/v5 migration item from the planner queue.
• Add Add end-to-end streaming chat conformance #4738 as the next material builder target: end-to-end streaming chat conformance.

Testing:

• go build ./...
• go test ./... (fails in existing environment: AtlasCloud stream endpoint returns 400 without expected credential isolation; loopback grpc tests are blocked by envoy)
• golangci-lint run ./... (fails because installed golangci-lint was built with Go 1.24 while module targets Go 1.25.12)

Closes #4737

[Copilot]

Pull request overview

Updates the autonomous improvement loop’s ranked work queue to reflect that the pgx/v5 migration/security-upkeep work is complete and to set the next top priority to streaming chat conformance work tracked in #4738.

Changes:

• Removes the previously top-ranked “pgx/v4 → pgx/v5” migration item from the queue.
• Adds “end-to-end streaming chat conformance” as the new top-ranked queue item, linking to issue #4738.

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